Reinsurance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 2,370,400	$ 2,114,100	$ 1,935,300
Ceded to Other Companies	953,100	904,800	840,500
Assumed from Other Companies	4,582,900	4,268,800	3,881,400
Net Amount	6,000,161	5,478,143	4,976,190
Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	2,370,400	2,114,100	1,935,300
Ceded to Other Companies	953,100	904,800	840,500
Assumed from Other Companies	4,582,900	4,268,800	3,881,400
Net Amount	$ 6,000,200	$ 5,478,100	$ 4,976,200
Percentage of Amount Assumed to Net	76.40%	77.90%	78.00%