Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	$ 36,131.1	$ 33,742.1
Derivative Assets	2,609.5	2,654.1
Net Amount	986.7	794.1
Derivative Liabilities	716.4	1,207.6
Derivative asset reduction as a result of cash collateral received from and deposited with counterparties	1,622.8	1,860.0
Gross amounts offset, total derivatives subject to a master netting arrangement	1,631.5	1,621.4
Cross Product Collateral Adjustment
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative asset reduction as a result of cash collateral received from and deposited with counterparties	390.6	427.6
Gross amounts offset, total derivatives subject to a master netting arrangement	399.3	189.0
U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	5,624.8	5,700.3
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	712.3	746.4
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	21,583.7	18,676.6
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	141.4	177.2
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,271.2	2,993.0
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	824.9	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	1,921.7	1,820.0
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,586.8	2,291.3
Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2.9	4.3
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	443.3	435.1
Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	18.1	22.3
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	30,500.0	28,000.0
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	36,131.1	33,742.1
Trading Account	1.1	0.5
Total Available for Sale and Trading Debt Securities	36,132.2	33,742.6
Fair Value, Measurements, Recurring | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	5,624.8	5,700.3
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	712.3	746.4
Fair Value, Measurements, Recurring | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	21,583.7	18,676.6
Fair Value, Measurements, Recurring | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	141.4	177.2
Fair Value, Measurements, Recurring | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,271.2	2,993.0
Fair Value, Measurements, Recurring | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	824.9	875.6
Fair Value, Measurements, Recurring | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	1,921.7	1,820.0
Fair Value, Measurements, Recurring | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,586.8	2,291.3
Fair Value, Measurements, Recurring | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2.9	4.3
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	443.3	435.1
Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	18.1	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	(1,622.8)	(1,860.0)
Net Amount	986.7	794.1
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	2,503.0	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	106.5	97.0
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	716.4	1,207.6
Derivative liabilities, subject to netting arrangements	(1,631.5)	(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	2,179.9	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	130.9	83.5
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	37.1	30.4
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	5,624.8	5,700.3
Total Available for Sale and Trading Debt Securities	5,624.8	5,700.3
Fair Value, Measurements, Recurring | Level 1 | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	5,624.8	5,700.3
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	30,503.4	28,037.5
Trading Account	1.1	0.5
Total Available for Sale and Trading Debt Securities	30,504.5	28,038.0
Fair Value, Measurements, Recurring | Level 2 | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	712.3	746.4
Fair Value, Measurements, Recurring | Level 2 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	21,583.7	18,676.6
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	141.4	177.2
Fair Value, Measurements, Recurring | Level 2 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,271.2	2,993.0
Fair Value, Measurements, Recurring | Level 2 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	824.9	875.6
Fair Value, Measurements, Recurring | Level 2 | Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	1,921.7	1,820.0
Fair Value, Measurements, Recurring | Level 2 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2,586.8	2,291.3
Fair Value, Measurements, Recurring | Level 2 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	443.3	435.1
Fair Value, Measurements, Recurring | Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	18.1	22.3
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	2,609.5	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	2,503.0	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	106.5	97.0
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	2,311.3	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	2,179.9	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	130.9	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	0.5	0.7
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	2.9	4.3
Total Available for Sale and Trading Debt Securities	2.9	4.3
Fair Value, Measurements, Recurring | Level 3 | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for Sale (Note)	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	36.6	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	$ 36.6	$ 29.7