Reinsurance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 3,005,700	$ 2,370,400	$ 2,114,100
Ceded to Other Companies	1,272,800	953,100	904,800
Assumed from Other Companies	5,364,800	4,582,900	4,268,800
Net Amount	7,097,653	6,000,161	5,478,143
Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	3,005,700	2,370,400	2,114,100
Ceded to Other Companies	1,272,800	953,100	904,800
Assumed from Other Companies	5,364,800	4,582,900	4,268,800
Net Amount	$ 7,097,700	$ 6,000,200	$ 5,478,100
Percentage of Amount Assumed to Net	75.60%	76.40%	77.90%