Reinsurance - Schedule of Effect of Reinsurance Arrangements in Consolidated Statement of Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Premiums Written
Premiums Written, Direct	$ 278,242	$ 237,303	$ 797,776	$ 702,491
Premiums Written, Ceded	(63,493)	(46,858)	(422,092)	(406,300)
Premiums Written, Net	214,749	190,445	375,684	296,191
Premiums Earned
Premiums Earned, Direct	254,982	231,617	731,489	690,165
Premiums Earned, Ceded	(116,752)	(107,755)	(338,197)	(342,529)
Net premiums earned	138,230	123,862	393,292	347,636
Losses and Loss Adjustment Expenses
Losses and Loss Adjustment Expenses, Direct	205,337	108,788	452,014	383,354
Losses and Loss Adjustment Expenses, Ceded	(85,619)	(38,736)	(185,245)	(176,864)
Losses and Loss Adjustment Expenses, Net	$ 119,718	$ 70,052	$ 266,769	$ 206,490