Reinsurance Recoverables After Allowance for Credit Losses (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Insurance [Abstract]
Reinsurance recoverables on paid losses		$ 134,400		$ 85,300
Ceded outstanding loss and LAE	[1]	2,056,100		1,703,700	$ 1,620,900		$ 1,583,900
Reinsurance recoverables, before allowance for credit losses		2,190,500		1,789,000
Allowance for credit losses		(2,900)	$ (2,600)	(7,900)	$ (6,900)	$ (6,900)	$ 0
Total		$ 2,187,575		$ 1,781,096

[1] Reinsurance recoverables in this table include only ceded loss and LAE reserves.