Reinsurance Recoverables After Allowance for Credit Losses (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Insurance [Abstract]
Reinsurance recoverables on paid losses		$ 83,000		$ 98,100
Ceded outstanding loss and LAE	[1]	1,620,900		1,583,900	$ 1,509,700	$ 1,857,400
Reinsurance recoverables, before allowance for credit losses		1,703,900		1,682,000
Allowance for credit losses		(6,900)	$ (6,900)	0
Total		$ 1,696,996		$ 1,681,962

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