Annual report pursuant to Section 13 and 15(d)

SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Additional Information (Details)

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SCHEDULE I - CONDENSED FINANCIAL INFORMATION OF REGISTRANT - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Parent Company [Member]      
Condensed Financial Statements, Captions [Line Items]      
Cash dividends paid $ 46,774,111 $ 39,139,897 $ 23,392,442