Reconciliation of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	$ 186,930,000	$ 250,500,000
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	151,600,000	75,100,000
Reclassifications from accumulated other comprehensive income	(59,500,000)	(12,100,000)
Total	92,100,000	63,000,000
Balance as of March 31	279,049,000	313,500,000
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	238,400,000	263,300,000
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	151,100,000	85,200,000
Reclassifications from accumulated other comprehensive income	(59,500,000)	(12,100,000)
Total	91,600,000	73,100,000
Balance as of March 31	330,000,000	336,400,000
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(49,300,000)	(13,400,000)
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	400,000	(10,000,000)
Total	400,000	(10,000,000)
Balance as of March 31	(48,900,000)	(23,400,000)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance as of January 1	(2,200,000)	600,000
Other comprehensive income, net of tax:
Other comprehensive income before reclassifications	100,000	(100,000)
Total	100,000	(100,000)
Balance as of March 31	$ (2,100,000)	$ 500,000