Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 9,142,031	$ 8,456,239	$ 8,755,720	$ 8,776,734
Other comprehensive income (loss), net of tax:
Ending Balance	8,859,510	8,596,395	8,859,510	8,596,395
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 (loss) income before reclassifications			(182,000)	212,200
Reclassifications from accumulated other comprehensive income	(16,000)	(6,300)	(38,200)	(22,800)
Total			(220,200)	189,400
Ending Balance	344,700	510,400	344,700	510,400
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 (loss) income before reclassifications			(3,200)	13,200
Reclassifications from accumulated other comprehensive income			0	0
Total			(3,200)	13,200
Ending Balance	(102,600)	(108,600)	(102,600)	(108,600)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(13,100)	(27,900)
Other comprehensive income (loss), net of tax:
Other comprehensive (loss) income before reclassifications			(700)	1,400
Reclassifications from accumulated other comprehensive income			0	0
Total			(700)	1,400
Ending Balance	(13,800)	(26,500)	(13,800)	(26,500)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	303,196	291,993	452,402	171,350
Other comprehensive income (loss), net of tax:
Other comprehensive (loss) income before reclassifications			(185,900)	226,800
Reclassifications from accumulated other comprehensive income			(38,200)	(22,800)
Total			(224,100)	204,000
Ending Balance	$ 228,349	$ 375,310	$ 228,349	$ 375,310