Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 8,755,720	$ 8,776,734
Other comprehensive income (loss), net of tax:
Ending Balance		9,186,882	8,755,720
Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(3,570)
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		564,900	321,000
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(253,300)	277,000
Reclassifications from accumulated other comprehensive income		(52,500)	(33,100)
Total		(305,800)	243,900
Ending Balance		259,100	564,900
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(99,400)	(121,800)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(4,300)	22,400
Reclassifications from accumulated other comprehensive income		0	0
Total		(4,300)	22,400
Ending Balance		(103,700)	(99,400)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	[1]	(13,100)	(27,900)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(500)	(400)	[1]
Reclassifications from accumulated other comprehensive income		0	15,200	[1]
Total		(500)	14,800	[1]
Ending Balance		(13,600)	(13,100)	[1]
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		452,402	171,350
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(258,100)	299,000
Reclassifications from accumulated other comprehensive income		(52,500)	(17,900)
Total		(310,600)	281,100
Ending Balance		$ 141,822	452,402
Accumulated Other Comprehensive Income (loss) | Cumulative Effect, Period of Adoption, Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 0

[1] See Note 15 for further information on retirement plan-related reclassifications from accumulated other comprehensive income.