Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 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	8,695,881	7,857,062
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	(217,900)	(276,600)
Reclassifications from accumulated other comprehensive income	(11,800)	(10,900)
Total	(229,700)	(287,500)
Ending Balance	335,200	33,500
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	2,500	(13,700)
Reclassifications from accumulated other comprehensive income	0	0
Total	2,500	(13,700)
Ending Balance	(96,900)	(135,500)
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	(1,000)	500
Reclassifications from accumulated other comprehensive income	0	0
Total	(1,000)	500
Ending Balance	(14,100)	(27,400)
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 (loss) income before reclassifications	(216,400)	(289,800)
Reclassifications from accumulated other comprehensive income	(11,800)	(10,900)
Total	(228,200)	(300,700)
Ending Balance	$ 224,190	$ (129,343)