Transactions with Affiliates - Schedule of Southern and Le Mars Assumed from Donegal Mutual Pursuant to Quota-Share Reinsurance Agreements (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Losses and loss expenses	$ 114,865,113	$ 102,124,332	$ 100,110,773
Liability for losses and loss expenses	356,384,000
Southern and Le Mars [Member]
Ceded Credit Risk [Line Items]
Premiums earned	(271)	(1,512)	880,787
Losses and loss expenses	(690,268)	(378,199)	1,492,673
Unearned premiums	0	0	0
Liability for losses and loss expenses	$ 827,193	$ 3,222,100	$ 5,722,000