Reinsurance - Additional Information (Detail) - USD ($)		3 Months Ended
	Mar. 01, 2018	Mar. 31, 2018
Ceded Credit Risk [Line Items]
Loss reinsurance, set retention amount		$ 1,000,000
Percentage of accumulation of losses		100.00%
Catastrophe reinsurance, set retention amount		$ 5,000,000
Catastrophe reinsurance, deductible amount		$ 5,000,000
Increase loss retention of reinsurance coverage	$ 250,000
Maximum [Member]
Ceded Credit Risk [Line Items]
Increase loss retention of reinsurance coverage	$ 1,000,000
Atlantic States [Member]
Ceded Credit Risk [Line Items]
Percentage of share in results of pooled business subsidiary		80.00%
Donegal Mutual [Member]
Ceded Credit Risk [Line Items]
Percentage of share in results of pooled business owned by third party		20.00%
Maximum amount of loss coverage under reinsurance agreement of property catastrophe		$ 170,000,000