Reinsurance Recoverables After Allowance for Credit Losses (Detail) - USD ($) $ in Thousands		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Reinsurance recoverables on paid losses		$ 96,500	$ 85,300
Ceded outstanding loss and LAE	[1]	1,738,700	1,703,700	$ 1,559,500	$ 1,583,900
Reinsurance recoverables, before allowance for credit losses		1,835,200	1,789,000
Allowance for credit losses		(6,000)	(7,900)	$ (6,800)	$ 0
Total		$ 1,829,187	$ 1,781,096

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