Transactions with Affiliates - Schedule of Reinsurance Atlantic States Assumed from Pool (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Premiums earned	$ 326,198,234	$ 310,179,267	$ 282,646,219
Liability for losses and loss expenses	441,714,000
Affiliated Entity [Member] | Atlantic States Assumed [Member]
Ceded Credit Risk [Line Items]
Premiums earned	473,512,781	451,470,894	422,985,921
Losses and loss expenses	335,789,280	289,503,373	240,394,302
Unearned premiums	231,958,181	228,988,598	214,372,048
Liability for losses and loss expenses	$ 303,546,744	$ 252,263,547	$ 230,543,393