Employee Retirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Other comprehensive loss reclassed to expenses	$ 48,581	$ 74,830	$ 15,732
Other comprehensive loss reclassed to expenses	2,429,262	2,263,326	1,579,309
Parent Company
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	3,900	44,100
Fair value of plan assets	3,800	3,500
Other comprehensive loss reclassed to expenses	48,849	70,692	$ 17,565
Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	57,000
Other comprehensive losses attributable to retirement plans	3,800	15,400
Executive Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income | Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Other comprehensive loss reclassed to expenses	19,200
Executive Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income | Retirement Plans | Reinsurance Segment
Defined Benefit Plan Disclosure [Line Items]
Other comprehensive loss reclassed to expenses	5,600
Executive Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income | Retirement Plans | Parent Company
Defined Benefit Plan Disclosure [Line Items]
Other comprehensive loss reclassed to expenses	13,600
Transatlantic Holdings Incorporated
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations	72,200	79,000
Fair value of plan assets	$ 63,700	$ 56,900