Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	$ 33,844.9	$ 35,579.8
Trading Account	0.8	0.3
Derivative Assets	3,217.2	3,856.8
Net Amount	843.7	1,686.4
Derivative Liabilities	1,190.6	944.9
Gross amounts offset, total derivatives subject to a master netting arrangement, assets	2,373.5	2,170.4
Gross amounts offset, total derivatives subject to a master netting arrangement, liabilities	2,089.8	2,431.2
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement, assets	617.9	461.3
Gross amounts offset, total derivatives subject to a master netting arrangement, liabilities	334.2	722.1
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	28,100.0	28,100.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	33,845.7	35,580.1
Fair Value, Measurements, Recurring | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	33,844.9	35,579.8
Trading Account	0.8	0.3
Fair Value, Measurements, Recurring | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,744.4	7,522.6
Fair Value, Measurements, Recurring | Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	765.9	885.2
Fair Value, Measurements, Recurring | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	18,082.8	17,892.8
Fair Value, Measurements, Recurring | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	138.7	417.9
Fair Value, Measurements, Recurring | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,181.3	3,765.2
Fair Value, Measurements, Recurring | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	832.1	1,143.9
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	2,202.8	1,340.7
Fair Value, Measurements, Recurring | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,394.3	2,085.1
Fair Value, Measurements, Recurring | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	4.1	4.7
Fair Value, Measurements, Recurring | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	466.5	471.6
Fair Value, Measurements, Recurring | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	32.0	50.1
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets, subject to netting arrangement	(2,373.5)	(2,170.4)
Net Amount	843.7	1,686.4
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	3,100.1	3,609.6
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	117.1	247.2
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	1,190.6	944.9
Derivative liabilities, subject to netting arrangements	(2,089.8)	(2,431.2)
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,168.6	3,242.9
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	84.1	108.0
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	27.7	25.2
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	5,744.4	7,522.6
Fair Value, Measurements, Recurring | Level 1 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,744.4	7,522.6
Fair Value, Measurements, Recurring | Level 1 | Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,744.4	7,522.6
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	28,097.2	28,052.8
Fair Value, Measurements, Recurring | Level 2 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	28,096.4	28,052.5
Trading Account	0.8	0.3
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	765.9	885.2
Fair Value, Measurements, Recurring | Level 2 | Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	18,082.8	17,892.8
Fair Value, Measurements, Recurring | Level 2 | Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	138.7	417.9
Fair Value, Measurements, Recurring | Level 2 | Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	3,181.3	3,765.2
Fair Value, Measurements, Recurring | Level 2 | Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	832.1	1,143.9
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	2,202.8	1,340.7
Fair Value, Measurements, Recurring | Level 2 | Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,394.3	2,085.1
Fair Value, Measurements, Recurring | Level 2 | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	0.0
Fair Value, Measurements, Recurring | Level 2 | Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	466.5	471.6
Fair Value, Measurements, Recurring | Level 2 | Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	32.0	50.1
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,217.2	3,856.8
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,100.1	3,609.6
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	117.1	247.2
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,252.7	3,350.9
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,168.6	3,242.9
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	84.1	108.0
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Securities	4.1	4.7
Fair Value, Measurements, Recurring | Level 3 | Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	4.1	4.7
Fair Value, Measurements, Recurring | Level 3 | Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	4.1	4.7
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	27.7	25.2
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	$ 27.7	$ 25.2