Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investment Holdings [Line Items]
Amortized Cost	$ 29,514.1	$ 28,382.3
Gross Unrealized Gains	110.9	174.2
Gross Unrealized Losses	66.5	163.7
Fair Value	29,558.5us-gaap_AvailableForSaleSecurities	28,392.8us-gaap_AvailableForSaleSecurities
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	4,493.5	1,896.7
Gross Unrealized Gains	15.1	22.6
Gross Unrealized Losses	1.7	1.4
Fair Value	4,506.9	1,917.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	4.5	4.5
Gross Unrealized Gains	0.1	0.1
Fair Value	4.6	4.6
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,326.4	17,495.2
Gross Unrealized Gains	82.3	80.7
Gross Unrealized Losses	19.5	47.9
Fair Value	16,389.2	17,528.0
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	309.5	307.0
Gross Unrealized Gains	0.9	3.6
Fair Value	310.4	310.6
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,617.5	3,615.2
Gross Unrealized Gains	1.8	10.5
Gross Unrealized Losses	41.6	101.2
Fair Value	3,577.7	3,524.5
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,899.9	1,898.9
Gross Unrealized Gains	7.9	50.9
Gross Unrealized Losses	0.3	5.9
Fair Value	1,907.5	1,943.9
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	360.0	410.0
Gross Unrealized Gains	1.5	1.7
Gross Unrealized Losses	0.9	1.7
Fair Value	360.6	410.0
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	6.9	52.4
Gross Unrealized Gains		0.1
Gross Unrealized Losses	0.5	4.4
Fair Value	6.4	48.1
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,321.8	2,390.8
Gross Unrealized Gains	0.5	1.4
Gross Unrealized Losses	1.0	0.4
Fair Value	2,321.3	2,391.8
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	18.4	97.5
Gross Unrealized Gains	0.5	2.2
Gross Unrealized Losses	0.8	0.8
Fair Value	18.1	98.9
Others
Investment Holdings [Line Items]
Amortized Cost	155.7	214.1
Gross Unrealized Gains	0.3	0.4
Gross Unrealized Losses	0.2
Fair Value	$ 155.8	$ 214.5

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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