Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	$ 353,584	$ 186,900
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	(207,300)	283,200
Reclassifications from accumulated other comprehensive income	(30,000)	(124,100)
Total	(237,300)	159,100
Balance as of September 30	116,333	346,000
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	455,400	238,400
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	(198,700)	302,900
Reclassifications from accumulated other comprehensive income	(30,000)	(124,100)
Total	(228,700)	178,800
Balance as of September 30	226,700	417,200
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(89,200)	(49,300)
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	(8,300)	(19,800)
Total	(8,300)	(19,800)
Balance as of September 30	(97,500)	(69,100)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(12,600)	(2,200)
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	(300)	100
Total	(300)	100
Balance as of September 30	$ (12,900)	$ (2,100)