Reconciliation of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1,	$ 116,273	$ 353,600
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	122,900	50,300
Reclassifications from accumulated other comprehensive income	(9,800)	5,900
Total	113,100	56,200
Balance as of March 31,	229,376	409,800
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1,	231,900	455,400
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	102,100	61,600
Reclassifications from accumulated other comprehensive income	(9,800)	5,900
Total	92,300	67,500
Balance as of March 31,	324,200	522,900
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1,	(104,000)	(89,200)
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	20,600	(10,800)
Total	20,600	(10,800)
Balance as of March 31,	(83,400)	(100,000)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1,	(11,600)	(12,600)
Other comprehensive income, net of tax:
Other comprehensive income (loss) before reclassifications	200	(500)
Total	200	(500)
Balance as of March 31,	$ (11,400)	$ (13,100)