Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 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	$ (17,400)	$ (10,100)	(57,700)	$ (42,700)
Ending Balance	8,201,023		8,201,023
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			364,600	238,400
Reclassifications from accumulated other comprehensive income			(57,700)	(42,700)
Total			306,900	195,700
Ending Balance	539,100	427,600	539,100	427,600
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			22,400	19,200
Total			22,400	19,200
Ending Balance	(88,800)	(84,800)	(88,800)	(84,800)
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			(200)	400
Total			(200)	400
Ending Balance	(11,900)	(11,200)	(11,900)	(11,200)
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			386,800	258,000
Reclassifications from accumulated other comprehensive income			(57,700)	(42,700)
Total			329,100	215,300
Ending Balance	$ 438,400	$ 331,600	$ 438,400	$ 331,600