Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 9,186,882	$ 8,755,720
Other comprehensive (loss) income, net of tax:
Ending Balance	8,595,373	8,695,881
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	259,100	564,900
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications	(614,300)	(217,900)
Reclassifications from accumulated other comprehensive income	8,400	(11,800)
Total	(605,900)	(229,700)
Ending Balance	(346,800)	335,200
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(103,700)	(99,400)
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications	(500)	2,500
Reclassifications from accumulated other comprehensive income	0	0
Total	(500)	2,500
Ending Balance	(104,200)	(96,900)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(13,600)	(13,100)
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications	100	(1,000)
Reclassifications from accumulated other comprehensive income	0	0
Total	100	(1,000)
Ending Balance	(13,500)	(14,100)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	141,822	452,402
Other comprehensive (loss) income, net of tax:
Other comprehensive (loss) income before reclassifications	(614,700)	(216,400)
Reclassifications from accumulated other comprehensive income	8,400	(11,800)
Total	(606,300)	(228,200)
Ending Balance	$ (464,460)	$ 224,190