Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 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	7,857,062	8,236,487
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	(276,600)	183,400
Reclassifications from accumulated other comprehensive income	(10,900)	4,400
Total	(287,500)	187,800
Ending Balance	33,500	126,200
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	(13,700)	(900)
Total	(13,700)	(900)
Ending Balance	(135,500)	(125,600)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(27,900)	(15,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	500	900
Total	500	900
Ending Balance	(27,400)	(14,800)
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	(289,800)	183,400
Reclassifications from accumulated other comprehensive income	(10,900)	4,400
Total	(300,700)	187,800
Ending Balance	$ (129,343)	$ (14,157)