Transactions with Affiliates - Schedule of Reinsurance Ceded to Donegal Mutual Pursuant to these Quota-Share Reinsurance Agreements (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
Prepaid reinsurance premiums	135,379,777	135,032,641
Donegal Mutual [Member]
Ceded Credit Risk [Line Items]
Premiums earned	42,813,929	42,578,047	39,917,800
Losses and loss expenses	23,175,456	24,978,631	21,524,856
Prepaid reinsurance premiums	19,047,084	19,827,115	19,180,421
Liability for losses and loss expenses	$ 38,434,078	$ 36,396,109	$ 31,881,756