Annual report pursuant to Section 13 and 15(d)

Income Taxes - Common and Preferred Stock Distribution (Details)

v3.8.0.1
Income Taxes - Common and Preferred Stock Distribution (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) $ 3.000 $ 3.000 $ 2.750
Dividend Declared [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 3.0000 3.0000 2.7500
Preferred Stock, Dividends declared per share (in dollars per share) 1.8436 1.8436 1.5569
Ordinary Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 2.3700 1.1820 1.9800
Preferred Stock, Dividends declared per share (in dollars per share) 1.8436 1.8436 1.5569
Qualified Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.3140 0.0000 0.0534
Preferred Stock, Dividends declared per share (in dollars per share) 0.2444 0.0000 0.0419
Capital Gain Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Nondividend Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.6300 1.8180 0.7700
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment One [Member] | Dividend Declared [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.7500 0.7500 0.6500
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.6351
Installment One [Member] | Ordinary Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.5925 0.2955 0.4680
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.6351
Installment One [Member] | Qualified Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0785 0.0000 0.0126
Preferred Stock, Dividends declared per share (in dollars per share) 0.0611 0.0000 0.0171
Installment One [Member] | Capital Gain Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment One [Member] | Nondividend Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.1575 0.4545 0.1820
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment Two [Member] | Dividend Declared [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.7500 0.7500 0.6750
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.4609
Installment Two [Member] | Ordinary Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.5925 0.2955 0.4860
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.4609
Installment Two [Member] | Qualified Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0785 0.0000 0.0131
Preferred Stock, Dividends declared per share (in dollars per share) 0.0611 0.0000 0.0124
Installment Two [Member] | Capital Gain Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment Two [Member] | Nondividend Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.1575 0.4545 0.1890
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment Three [Member] | Dividend Declared [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.7500 0.7500 0.6750
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.4609
Installment Three [Member] | Ordinary Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.5925 0.2955 0.4860
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609 0.4609
Installment Three [Member] | Qualified Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0785 0.0000 0.0131
Preferred Stock, Dividends declared per share (in dollars per share) 0.0611 0.0000 0.0124
Installment Three [Member] | Capital Gain Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment Three [Member] | Nondividend Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.1575 0.4545 0.1890
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Installment Four [Member] | Dividend Declared [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.7500 0.7500 0.7500
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609  
Installment Four [Member] | Ordinary Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.5925 0.2955 0.5400
Preferred Stock, Dividends declared per share (in dollars per share) 0.4609 0.4609  
Installment Four [Member] | Qualified Dividends [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0785 0.0000 0.0146
Preferred Stock, Dividends declared per share (in dollars per share) 0.0611 0.0000  
Installment Four [Member] | Capital Gain Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000 0.0000
Preferred Stock, Dividends declared per share (in dollars per share) 0.0000 0.0000  
Installment Four [Member] | Nondividend Distribution [Member]      
Dividends Payable [Line Items]      
Common Stock, Dividends declared per share (in dollars per share) 0.1575 0.4545 $ 0.2100
Preferred Stock, Dividends declared per share (in dollars per share) $ 0.0000 $ 0.0000  
Common Stock [Member]      
Dividends Payable [Line Items]      
Ordinary income dividend percentage 79.00%    
Return of capital percentage 21.00%    
Qualified dividend income percentage 13.20%    
Preferred Stock [Member]      
Dividends Payable [Line Items]      
Ordinary income dividend percentage 100.00%    
Return of capital percentage 0.00%    
Qualified dividend income percentage 13.30%