Reconciliation of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	$ 353,584,000	$ 186,900,000
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	50,300,000	151,600,000
Reclassifications from accumulated other comprehensive income	5,900,000	(59,500,000)
Total	56,200,000	92,100,000
Balance as of March 31	409,789,000	279,000,000
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	455,400,000	238,400,000
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	61,600,000	151,100,000
Reclassifications from accumulated other comprehensive income	5,900,000	(59,500,000)
Total	67,500,000	91,600,000
Balance as of March 31	522,900,000	330,000,000
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(89,200,000)	(49,300,000)
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	(10,800,000)	400,000
Total	(10,800,000)	400,000
Balance as of March 31	(100,000,000)	(48,900,000)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(12,600,000)	(2,200,000)
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	(500,000)	100,000
Total	(500,000)	100,000
Balance as of March 31	$ (13,100,000)	$ (2,100,000)