Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pre tax total accumulated other comprehensive income, beginning balance	$ 1,463,500,000		$ 1,491,800,000		$ 1,464,100,000		$ 1,340,000,000
Investment securities, pre tax	188,900,000		(147,100,000)		320,700,000		80,700,000
Net non-credit related OTTI losses, adjusted for valuation changes, pre tax	0		100,000		0		400,000
Forecasted transactions, pre tax	(1,600,000)		0		(1,600,000)		0
Foreign currency translation adjustment, pre tax	800,000		(1,200,000)		800,000		(1,500,000)
Total other comprehensive income (loss) before reclassifications, pre tax	188,100,000		(148,200,000)		319,900,000		79,600,000
Total reclassification adjustment for amounts realized in net income, pre tax	53,400,000		91,000,000		185,800,000		167,000,000
Total other comprehensive income (loss), pre tax	134,700,000		(239,200,000)		134,100,000		(87,400,000)
Pre tax total accumulated other comprehensive income, ending balance	1,598,200,000		1,252,600,000		1,598,200,000		1,252,600,000
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Accumulated other comprehensive income loss tax (Provision) benefit, beginning balance	(512,200,000)		(522,100,000)		(512,400,000)		(469,000,000)
Investment securities, Total tax (provision) benefit	(66,100,000)		51,500,000		(112,200,000)		(28,200,000)
Net non-credit related OTTI losses, adjusted for valuation changes, Total tax (provision) benefit	0		0		0		(100,000)
Forecasted transactions, Total tax (provision) benefit	600,000		0		600,000		0
Foreign currency translation adjustment, Total tax (provision) benefit	(300,000)		400,000		(300,000)		500,000
Total other comprehensive income (loss) before reclassifications, Total tax (provision) benefit	(65,800,000)		51,900,000		(111,900,000)		(27,800,000)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(18,700,000)		(31,800,000)		(65,000,000)		(58,400,000)
Total other comprehensive income (loss), Total tax (provision) benefit	(47,100,000)		83,700,000		(46,900,000)		30,600,000
Accumulated other comprehensive income loss tax (Provision) benefit, ending balance	(559,300,000)		(438,400,000)		(559,300,000)		(438,400,000)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	951,300,000		969,700,000		951,700,000		871,000,000
Investment securities, after tax	122,800,000		(95,600,000)		208,500,000		52,500,000
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0		100,000		0		300,000
Forecasted transactions, after tax	(1,000,000)		0		(1,000,000)		0
Foreign currency translation adjustment, after tax	500,000		(800,000)		500,000		(1,000,000)
Total other comprehensive income (loss) before reclassifications, after tax	122,300,000		(96,300,000)		208,000,000		51,800,000
Total reclassification adjustment for amounts realized in net income, after tax	34,700,000		59,200,000		120,800,000		108,600,000
Other comprehensive income (loss)	87,600,000		(155,500,000)		87,200,000		(56,800,000)
After tax total accumulated other comprehensive income, ending balance	1,038,900,000		814,200,000		1,038,900,000		814,200,000
Net impairment losses recognized in earnings	0		(1,600,000)		0		(1,800,000)
Net realized gains (losses) on securities	40,400,000		134,500,000		159,800,000		215,300,000
Interest expense	29,600,000		30,400,000		56,300,000		61,000,000
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	946,900,000		961,900,000		947,000,000		862,700,000
Investment securities, after tax	122,800,000		(95,600,000)		208,500,000		52,500,000
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0		100,000		0		300,000
Forecasted transactions, after tax	0		0		0		0
Foreign currency translation adjustment, after tax	0		0		0		0
Total other comprehensive income (loss) before reclassifications, after tax	122,800,000		(95,500,000)		208,500,000		52,800,000
Total reclassification adjustment for amounts realized in net income, after tax	34,400,000		58,800,000		120,200,000		107,900,000
Other comprehensive income (loss)	88,400,000		(154,300,000)		88,300,000		(55,100,000)
After tax total accumulated other comprehensive income, ending balance	1,035,300,000		807,600,000		1,035,300,000		807,600,000
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	3,800,000	[1],[2]	5,800,000	[1]	4,100,000	[1],[2]	6,100,000	[1]
Investment securities, after tax	0	[1],[2]	0	[1]	0	[1],[2]	0	[1]
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0	[1],[2]	0	[1]	0	[1],[2]	0	[1]
Forecasted transactions, after tax	(1,000,000)	[1],[2]	0	[1]	(1,000,000)	[1],[2]	0	[1]
Foreign currency translation adjustment, after tax	0	[1],[2]	0	[1]	0	[1],[2]	0	[1]
Total other comprehensive income (loss) before reclassifications, after tax	(1,000,000)	[1],[2]	0	[1]	(1,000,000)	[1],[2]	0	[1]
Total reclassification adjustment for amounts realized in net income, after tax	300,000	[1],[2]	400,000	[1]	600,000	[1],[2]	700,000	[1]
Other comprehensive income (loss)	(1,300,000)	[1],[2]	(400,000)	[1]	(1,600,000)	[1],[2]	(700,000)	[1]
After tax total accumulated other comprehensive income, ending balance	2,500,000	[1],[2]	5,400,000	[1]	2,500,000	[1],[2]	5,400,000	[1]
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	600,000		2,000,000		600,000		2,200,000
Investment securities, after tax	0		0		0		0
Net non-credit related OTTI losses, adjusted for valuation changes, after tax	0		0		0		0
Forecasted transactions, after tax	0		0		0		0
Foreign currency translation adjustment, after tax	500,000		(800,000)		500,000		(1,000,000)
Total other comprehensive income (loss) before reclassifications, after tax	500,000		(800,000)		500,000		(1,000,000)
Total reclassification adjustment for amounts realized in net income, after tax	0		0		0		0
Other comprehensive income (loss)	500,000		(800,000)		500,000		(1,000,000)
After tax total accumulated other comprehensive income, ending balance	1,100,000		1,200,000		1,100,000		1,200,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income pretax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0		(1,500,000)		0		(1,600,000)
Net realized gains (losses) on securities	52,900,000		92,000,000		184,800,000		167,600,000
Interest expense	500,000	[2]	500,000		1,000,000	[2]	1,000,000
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0		500,000		0		600,000
Net realized gains (losses) on securities	(18,500,000)		(32,200,000)		(64,600,000)		(58,700,000)
Interest expense	(200,000)	[2]	(100,000)		(400,000)	[2]	(300,000)
Reclassification out of Accumulated Other Comprehensive Income | Accumulated other comprehensive income after tax
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0		(1,000,000)		0		(1,000,000)
Net realized gains (losses) on securities	34,400,000		59,800,000		120,200,000		108,900,000
Interest expense	300,000	[2]	400,000		600,000	[2]	700,000
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0		(1,000,000)		0		(1,000,000)
Net realized gains (losses) on securities	34,400,000		59,800,000		120,200,000		108,900,000
Interest expense	0	[2]	0		0	[2]	0
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0	[1],[2]	0	[1]	0	[1],[2]	0	[1]
Net realized gains (losses) on securities	0	[1],[2]	0	[1]	0	[1],[2]	0	[1]
Interest expense	300,000	[1],[2]	400,000	[1]	600,000	[1],[2]	700,000	[1]
Reclassification out of Accumulated Other Comprehensive Income | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net impairment losses recognized in earnings	0		0		0		0
Net realized gains (losses) on securities	0		0		0		0
Interest expense	$ 0	[2]	$ 0		$ 0	[2]	$ 0

[1] Entered into for the purpose of managing interest rate risk associated with our debt issuances.

[2] We expect to reclassify $2.2 million (pretax) into income during the next 12 months, related to the net unrealized gains on forecasted transactions.