Property and Casualty Premiums Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Written And Earned [Abstract]
Gross premiums written – direct	$ 2,214,600	$ 1,800,300	$ 1,537,800
Gross premiums written – assumed	5,114,800	4,856,100	4,357,800
Ceded premiums written	(985,000)	(904,700)	(847,200)
Net premiums written	6,344,400	5,751,700	5,048,400
Gross premiums earned – direct	2,370,400	2,114,100	1,935,300
Gross premiums earned – assumed	4,582,900	4,268,800	3,881,400
Ceded premiums earned	(953,100)	(904,800)	(840,500)
Net premiums earned	$ 6,000,161	$ 5,478,143	$ 4,976,190