Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		$ 8,776,734		$ 7,692,710
Other comprehensive income (loss), net of tax:
Ending Balance		8,755,720		8,776,734
Cumulative Effect, Period of Adoption, Adjusted Balance
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(3,570)
Other comprehensive income (loss), net of tax:
Ending Balance				(3,570)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		171,350		(202,003)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		299,000		393,700
Reclassifications from accumulated other comprehensive income		(17,900)		(20,400)
Total		281,100		373,300
Ending Balance		452,402		171,350
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		321,000		(61,600)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		277,000		403,000
Reclassifications from accumulated other comprehensive income		(33,100)		(20,400)
Total		243,900		382,600
Ending Balance		564,900		321,000
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(121,800)		(124,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		22,400		2,900
Reclassifications from accumulated other comprehensive income		0		0
Total		22,400		2,900
Ending Balance		(99,400)		(121,800)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(27,900)	[1]	(15,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications		(400)	[1]	(12,200)
Reclassifications from accumulated other comprehensive income		15,200	[1]	0
Total		14,800	[1]	(12,200)
Ending Balance	[1]	$ (13,100)		$ (27,900)

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