Transactions with Affiliates - Schedule of Reinsurance Atlantic States Assumed from Pool (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Premiums earned	$ 310,179,267	$ 282,646,219	$ 271,712,289
Liability for losses and loss expenses	356,384,000
Affiliated Entity [Member] | Atlantic States Assumed [Member]
Ceded Credit Risk [Line Items]
Premiums earned	451,470,894	422,985,921	396,098,036
Losses and loss expenses	289,503,373	240,394,302	240,197,659
Unearned premiums	228,988,598	214,372,048	199,966,888
Liability for losses and loss expenses	$ 252,263,547	$ 230,543,393	$ 216,194,945