Reinsurance - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2019 USD ($)
Ceded Credit Risk [Line Items]
Loss reinsurance, set retention amount for property losses	$ 1,000,000
Loss reinsurance, set retention amount for casualty losses	$ 2,000,000
Third Party Reinsurance [Member]
Ceded Credit Risk [Line Items]
Percentage of accumulation of losses	100.00%
Catastrophe reinsurance, set retention amount	$ 10,000,000
Catastrophe reinsurance, deductible amount	1,200,000
Maximum amount of loss coverage under reinsurance agreement of property catastrophe	$ 190,000,000
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%
Percentage of accumulation of losses	100.00%
Catastrophe reinsurance, set retention amount	$ 2,000,000
Maximum amount of loss coverage under reinsurance agreement of property catastrophe	8,000,000
Additional coverage amount	$ 5,000,000