Property and Casualty Premiums Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Premiums Written And Earned [Abstract]
Gross premiums written – direct	$ 1,800,300	$ 1,537,800	$ 1,528,700
Gross premiums written – assumed	4,856,100	4,357,800	4,168,200
Ceded premiums written	(904,700)	(847,200)	(731,000)
Net premiums written	5,751,700	5,048,400	4,965,900
Gross premiums earned – direct	2,114,100	1,935,300	1,932,000
Gross premiums earned – assumed	4,268,800	3,881,400	3,790,300
Ceded premiums earned	(904,800)	(840,500)	(767,300)
Net premiums earned	$ 5,478,143	$ 4,976,190	$ 4,954,990