Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 30,814.6	$ 29,514.1
Gross Unrealized Gains	160.8	110.9
Gross Unrealized Losses	29.7	66.5
Fair Value	30,945.7us-gaap_AvailableForSaleSecurities	29,558.5us-gaap_AvailableForSaleSecurities
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	4,593.9	4,493.5
Gross Unrealized Gains	33.5	15.1
Gross Unrealized Losses		1.7
Fair Value	4,627.4	4,506.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	4.5	4.5
Gross Unrealized Gains		0.1
Fair Value	4.5	4.6
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,536.9	16,326.4
Gross Unrealized Gains	107.6	82.3
Gross Unrealized Losses	10.3	19.5
Fair Value	16,634.2	16,389.2
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	310.5	309.5
Gross Unrealized Gains	0.9	0.9
Fair Value	311.4	310.4
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,661.5	3,617.5
Gross Unrealized Gains	6.7	1.8
Gross Unrealized Losses	16.9	41.6
Fair Value	3,651.3	3,577.7
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	2,040.5	1,899.9
Gross Unrealized Gains	8.8	7.9
Gross Unrealized Losses		0.3
Fair Value	2,049.3	1,907.5
Sub-Sovereign, Supranational and Non US Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	560.0
Gross Unrealized Gains	1.4
Gross Unrealized Losses	0.4
Fair Value	561.0
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost		360.0
Gross Unrealized Gains		1.5
Gross Unrealized Losses		0.9
Fair Value		360.6
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	4.9	6.9
Gross Unrealized Losses	0.5	0.5
Fair Value	4.4	6.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,948.7	2,321.8
Gross Unrealized Gains	1.1	0.5
Gross Unrealized Losses	0.5	1.0
Fair Value	2,949.3	2,321.3
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	18.1	18.4
Gross Unrealized Gains	0.5	0.5
Gross Unrealized Losses	1.1	0.8
Fair Value	17.5	18.1
Others
Investment Holdings [Line Items]
Amortized Cost	135.1	155.7
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses		0.2
Fair Value	$ 135.4	$ 155.8

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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