Liability for Losses and Loss Expenses - Additional Information (Detail) - USD ($) $ in Thousands |
9 Months Ended | |
|---|---|---|
Sep. 30, 2018 |
Sep. 30, 2017 |
|
| Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] | ||
| Increase (Decrease) in liability for losses and loss expenses of prior years | $ 28,913 | $ 5,014 |
| Percentage of 2010 development | 7.50% | 1.40% |
| Personal Automobile [Member] | ||
| Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] | ||
| Reserve created for commercial automobile | $ 13,000 | |
| Commercial Automobile [Member] | ||
| Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] | ||
| Reserve created for commercial automobile | $ 19,100 | |