Transactions with Affiliates - Schedule of Reinsurance Ceded to Donegal Mutual Pursuant to these Quota-Share Reinsurance Agreements (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
Donegal Mutual [Member]
Ceded Credit Risk [Line Items]
Premiums earned	42,578,047	39,917,800	37,299,760
Losses and loss expenses	24,978,631	21,524,856	19,735,479
Prepaid reinsurance premiums	19,827,115	19,180,421	17,172,112
Liability for losses and loss expenses	$ 36,396,109	$ 31,881,756	$ 29,968,948