Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	$ 31,091.7	$ 29,558.5
Trading Account	0.5	4.7
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	31,092.2	29,563.2
Fair Value, Measurements, Recurring | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	31,091.7	29,558.5
Trading Account	0.5	4.7
Fair Value, Measurements, Recurring | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,125.1	4,506.9
Fair Value, Measurements, Recurring | Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	4.5	4.6
Fair Value, Measurements, Recurring | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	15,705.9	16,389.2
Fair Value, Measurements, Recurring | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	311.0	310.4
Fair Value, Measurements, Recurring | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,794.6	3,577.7
Fair Value, Measurements, Recurring | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,073.5	1,907.5
Fair Value, Measurements, Recurring | Securities | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value		360.6
Fair Value, Measurements, Recurring | Securities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value		6.4
Fair Value, Measurements, Recurring | Securities | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	836.1
Fair Value, Measurements, Recurring | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,815.7	2,321.3
Fair Value, Measurements, Recurring | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.2	18.1
Fair Value, Measurements, Recurring | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	299.4
Fair Value, Measurements, Recurring | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	109.7	155.8
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	1,934.1	2,250.4
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	2,818.3	4,275.2
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	282.8	232.3
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	689.7	1,054.0
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	2,774.6	4,095.5
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	165.5	131.8
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives (1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	10.9
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	5,125.1	4,506.9
Fair Value, Measurements, Recurring | Level 1 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,125.1	4,506.9
Fair Value, Measurements, Recurring | Level 1 | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,125.1	4,506.9
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	25,950.9	25,038.2
Fair Value, Measurements, Recurring | Level 2 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	25,950.4	25,033.5
Trading Account	0.5	4.7
Fair Value, Measurements, Recurring | Level 2 | Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	4.5	4.6
Fair Value, Measurements, Recurring | Level 2 | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	15,705.9	16,389.2
Fair Value, Measurements, Recurring | Level 2 | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	311.0	310.4
Fair Value, Measurements, Recurring | Level 2 | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,794.6	3,577.7
Fair Value, Measurements, Recurring | Level 2 | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,073.5	1,907.5
Fair Value, Measurements, Recurring | Level 2 | Securities | Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value		360.6
Fair Value, Measurements, Recurring | Level 2 | Securities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value		6.4
Fair Value, Measurements, Recurring | Level 2 | Securities | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	836.1
Fair Value, Measurements, Recurring | Level 2 | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,815.7	2,321.3
Fair Value, Measurements, Recurring | Level 2 | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	299.4
Fair Value, Measurements, Recurring | Level 2 | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	109.7	155.8
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	3,101.1	4,507.5
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	2,818.3	4,275.2
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	282.8	232.3
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	2,940.1	4,227.3
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	2,774.6	4,095.5
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	165.5	131.8
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	16.2	18.1
Fair Value, Measurements, Recurring | Level 3 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.2	18.1
Fair Value, Measurements, Recurring | Level 3 | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.2	18.1
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	10.9
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities | Other Financial Derivatives (1)
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	10.9
Fair Value, Measurements, Recurring | Netting | Derivative Financial Instruments, Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	69.0	315.8
Fair Value, Measurements, Recurring | Netting | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	(1,167.0)	(2,257.1)
Fair Value, Measurements, Recurring | Netting | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(2,261.3)	(3,173.3)
Derivative Financial Instruments, Liabilities | Fair Value, Measurements, Recurring | Netting
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Reduction in derivatives due to cash collateral received from and deposited with derivative counterparties	$ 1,200.0	$ 1,200.0