Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 7,939,945
Other comprehensive income (loss), net of tax:
Reclassifications from accumulated other comprehensive income	(36,700)	$ (9,800)
Ending Balance	8,184,849
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	232,200	231,900
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	124,900	102,100
Reclassifications from accumulated other comprehensive income	(36,700)	(9,800)
Total	88,200	92,300
Ending Balance	320,400	324,200
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(111,200)	(104,000)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	5,100	20,600
Total	5,100	20,600
Ending Balance	(106,100)	(83,400)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(11,700)	(11,600)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	(400)	200
Total	(400)	200
Ending Balance	(12,100)	(11,400)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	109,300	116,300
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	129,600	122,900
Reclassifications from accumulated other comprehensive income	(36,700)	(9,800)
Total	92,900	113,100
Ending Balance	$ 202,200	$ 229,400