Transactions with Affiliates - Schedule of Insurance Subsidiaries Ceded to Donegal Mutual Pursuant to these 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
Insurance Subsidiaries [Member]
Ceded Credit Risk [Line Items]
Premiums earned	260,545,919	238,327,677	221,953,918
Catastrophe Reinsurance [Member] | Insurance Subsidiaries [Member]
Ceded Credit Risk [Line Items]
Premiums earned	17,215,273	12,965,868	14,235,227
Losses and loss expenses	8,953,411	995,076	6,814,836
Liability for losses and loss expenses	$ 3,399,207	$ 3,136,438	$ 4,485,201