Transactions with Affiliates - Schedule of Reinsurance Atlantic States Ceded to 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
Prepaid reinsurance premiums	135,032,641	124,255,495
Affiliated Entity [Member] | Atlantic States Ceded [Member]
Ceded Credit Risk [Line Items]
Premiums earned	200,752,599	185,444,009	170,418,931
Losses and loss expenses	140,015,950	115,371,839	115,029,244
Prepaid reinsurance premiums	103,991,861	95,469,329	87,780,338
Liability for losses and loss expenses	$ 136,786,070	$ 120,434,535	$ 108,672,769