Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	$ 33,330.2	$ 32,317.9
Trading Account	0.2	1.2
Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	10.4	10.9
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	26,500.0	26,100.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	33,330.4	32,319.1
Fair Value, Measurements, Recurring | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	33,330.2	32,317.9
Trading Account	0.2	1.2
Fair Value, Measurements, Recurring | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	6,857.1	6,178.3
Fair Value, Measurements, Recurring | Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	189.8	36.4
Fair Value, Measurements, Recurring | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16,892.6	16,366.8
Fair Value, Measurements, Recurring | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	280.7	309.5
Fair Value, Measurements, Recurring | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,857.8	3,712.2
Fair Value, Measurements, Recurring | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	1,374.8	1,870.2
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		859.4
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	1,070.2
Fair Value, Measurements, Recurring | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,300.1	2,500.1
Fair Value, Measurements, Recurring | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.6	17.1
Fair Value, Measurements, Recurring | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	406.9	374.4
Fair Value, Measurements, Recurring | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	83.6	93.5
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	1,730.1	1,251.7
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	3,952.6	2,623.4
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	334.0	228.5
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	1,337.9	943.9
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	4,028.4	2,519.4
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	215.1	131.2
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	10.4	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	6,857.1	6,178.3
Fair Value, Measurements, Recurring | Level 1 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	6,857.1	6,178.3
Fair Value, Measurements, Recurring | Level 1 | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	6,857.1	6,178.3
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	26,456.7	26,123.7
Fair Value, Measurements, Recurring | Level 2 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	26,456.5	26,122.5
Trading Account	0.2	1.2
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	189.8	36.4
Fair Value, Measurements, Recurring | Level 2 | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16,892.6	16,366.8
Fair Value, Measurements, Recurring | Level 2 | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	280.7	309.5
Fair Value, Measurements, Recurring | Level 2 | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,857.8	3,712.2
Fair Value, Measurements, Recurring | Level 2 | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	1,374.8	1,870.2
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		859.4
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	1,070.2
Fair Value, Measurements, Recurring | Level 2 | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,300.1	2,500.1
Fair Value, Measurements, Recurring | Level 2 | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	406.9	374.4
Fair Value, Measurements, Recurring | Level 2 | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	83.6	93.5
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	4,286.6	2,851.9
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	3,952.6	2,623.4
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	334.0	228.5
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	4,244.2	2,650.7
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	4,028.4	2,519.4
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	215.1	131.2
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	0.7	0.1
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	16.6	17.1
Fair Value, Measurements, Recurring | Level 3 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.6	17.1
Fair Value, Measurements, Recurring | Level 3 | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	16.6	17.1
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	9.7	10.8
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	9.7	10.8
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	871.9	322.8
Fair Value, Measurements, Recurring | Netting | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets	(2,556.5)	(1,600.2)
Fair Value, Measurements, Recurring | Netting | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative liabilities	(2,916.0)	(1,717.6)
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	$ 440.3