Reinsurance - Additional Information (Detail) - USD ($) |
9 Months Ended | |
|---|---|---|
Mar. 01, 2018 |
Sep. 30, 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 |