Reserve for Unpaid Losses And Loss Adjustment Expenses - Summary of Reserve for Unpaid Losses And LAE (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Insurance [Abstract]
Balance, beginning of period	$ 620,718	$ 430,413	$ 613,533	$ 432,359
Less: reinsurance recoverable on unpaid losses	404,370	223,023	393,630	250,506
Net balance, beginning of period	216,348	207,390	219,903	181,853
Incurred related to:
Current year	125,544	70,448	281,698	208,796
Prior years	(5,826)	(396)	(14,929)	(2,306)
Total incurred	119,718	70,052	266,769	206,490
Paid related to:
Current year	71,955	50,920	148,563	94,075
Prior years	18,371	17,820	92,369	85,566
Total paid	90,326	68,740	240,932	179,641
Net balance, end of period	245,740	208,702	245,740	208,702
Plus: reinsurance recoverable on unpaid losses	417,257	208,884	417,257	208,884
Balance, end of period	$ 662,997	$ 417,586	$ 662,997	$ 417,586