Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	$ 34,258.0	$ 33,742.1
Trading Account	8.9	0.5
Derivative Assets	2,142.5	2,654.1
Net Amount	981.7	794.1
Derivative Liabilities	659.4	1,207.6
Gross amounts offset, total derivatives subject to a master netting arrangement, assets	1,160.8	1,860.0
Gross amounts offset, total derivatives subject to a master netting arrangement, liabilities	1,529.7	1,621.4
Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement, assets	0.0
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	74.9	427.6
Gross amounts offset, total derivatives subject to a master netting arrangement, liabilities	443.8	189.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	28,600.0	28,000.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Debt Securities	34,266.9	33,742.6
Fair Value, Measurements, Recurring | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	34,258.0	33,742.1
Trading Account	8.9	0.5
Fair Value, Measurements, Recurring | Debt Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,651.7	5,700.3
Fair Value, Measurements, Recurring | Debt Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	687.7	746.4
Fair Value, Measurements, Recurring | Debt Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	19,554.7	18,676.6
Fair Value, Measurements, Recurring | Debt Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	141.0	177.2
Fair Value, Measurements, Recurring | Debt Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,706.7	2,993.0
Fair Value, Measurements, Recurring | Debt Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	842.2	875.6
Fair Value, Measurements, Recurring | Debt Securities | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	1,983.0	1,820.0
Fair Value, Measurements, Recurring | Debt Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,224.2	2,291.3
Fair Value, Measurements, Recurring | Debt Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2.9	4.3
Fair Value, Measurements, Recurring | Debt Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	442.9	435.1
Fair Value, Measurements, Recurring | Debt Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	21.0	22.3
Fair Value, Measurements, Recurring | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative assets, subject to netting arrangement	(1,160.8)	(1,860.0)
Net Amount	981.7	794.1
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	2,052.6	2,557.1
Fair Value, Measurements, Recurring | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	89.7	97.0
Fair Value, Measurements, Recurring | Other Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	0.2
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	659.4	1,207.6
Derivative liabilities, subject to netting arrangements	(1,529.7)	(1,621.4)
Fair Value, Measurements, Recurring | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	2,059.1	2,715.1
Fair Value, Measurements, Recurring | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	98.7	83.5
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	31.3	30.4
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Debt Securities	5,651.7	5,700.3
Fair Value, Measurements, Recurring | Level 1 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,651.7	5,700.3
Fair Value, Measurements, Recurring | Level 1 | Debt Securities | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	5,651.7	5,700.3
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Debt Securities	28,612.3	28,038.0
Fair Value, Measurements, Recurring | Level 2 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	28,603.4	28,037.5
Trading Account	8.9	0.5
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	687.7	746.4
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	19,554.7	18,676.6
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	141.0	177.2
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,706.7	2,993.0
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	842.2	875.6
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	1,983.0	1,820.0
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2,224.2	2,291.3
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	0.0
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	442.9	435.1
Fair Value, Measurements, Recurring | Level 2 | Debt Securities | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	21.0	22.3
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	2,142.5	2,654.1
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	2,052.6	2,557.1
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	89.7	97.0
Fair Value, Measurements, Recurring | Level 2 | Other Assets | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	0.2
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	2,157.8	2,799.3
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	2,059.1	2,715.1
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities | Interest Rate Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	98.7	83.5
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	0.7
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total Available for Sale and Trading Debt Securities	2.9	4.3
Fair Value, Measurements, Recurring | Level 3 | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2.9	4.3
Fair Value, Measurements, Recurring | Level 3 | Debt Securities | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Fair Value	2.9	4.3
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	31.3	29.7
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	$ 31.3	$ 29.7