Reserve for Unpaid Losses - Summary of Reserve for Unpaid Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Insurance [Abstract]
Balance, beginning of period	$ 637,882	$ 607,177	$ 659,341	$ 613,533
Less: reinsurance recoverable on unpaid losses	366,412	387,637	397,688	393,630
Net balance, beginning of period	271,470	219,540	261,653	219,903
Incurred related to:
Current year	101,445	83,822	200,949	156,153
Prior years	(611)	(4,953)	(2,206)	(9,103)
Total incurred	100,834	78,869	198,743	147,050
Paid related to:
Current year	69,795	55,372	95,621	76,608
Prior years	43,409	26,689	105,675	73,997
Total paid	113,204	82,061	201,296	150,605
Net balance, end of period	259,100	216,348	259,100	216,348
Plus: reinsurance recoverable on unpaid losses	366,879	404,370	366,879	404,370
Balance, end of period	$ 625,979	$ 620,718	$ 625,979	$ 620,718