Property and Casualty Premiums Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Premiums Written and Earned [Abstract]
Gross premiums written – direct	$ 2,704,300	$ 2,214,600	$ 1,800,300
Gross premiums written – assumed	5,834,500	5,114,800	4,856,100
Ceded premiums written	(1,389,400)	(985,000)	(904,700)
Net premiums written	7,149,400	6,344,400	5,751,700
Gross premiums earned – direct	3,005,700	2,370,400	2,114,100
Gross premiums earned – assumed	5,364,800	4,582,900	4,268,800
Ceded premiums earned	(1,272,800)	(953,100)	(904,800)
Net premiums earned	$ 7,097,653	$ 6,000,161	$ 5,478,143