Fair Value Measurements (Recurring Basis Hierarchy Leveling) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	$ 35,184.5	$ 33,742.1
Derivative Assets	3,409.4	2,654.1
Net Amount	1,633.6	794.1
Derivative Liabilities	749.9	1,207.6
Derivative asset reduction as a result of cash collateral received from and deposited with counterparties	1,775.8	1,860.0
Gross amounts offset, total derivatives subject to a master netting arrangement	2,463.3	1,621.4
Other Financial Derivatives
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	0.0
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	298.1	427.6
Gross amounts offset, total derivatives subject to a master netting arrangement	985.6	189.0
U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	5,638.5	5,700.3
Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	737.8	746.4
Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	21,054.5	18,676.6
Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	141.2	177.2
Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2,466.2	2,993.0
Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	831.9	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2,009.0	1,820.0
Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	1,855.8	2,291.3
Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2.8	4.3
Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	427.7	435.1
Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	19.1	22.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	35,184.5	33,742.1
Trading Account	0.7	0.5
Total Available for Sale and Trading Debt Securities	35,185.2	33,742.6
Fair Value, Measurements, Recurring | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	5,638.5	5,700.3
Fair Value, Measurements, Recurring | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	737.8	746.4
Fair Value, Measurements, Recurring | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	21,054.5	18,676.6
Fair Value, Measurements, Recurring | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	141.2	177.2
Fair Value, Measurements, Recurring | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2,466.2	2,993.0
Fair Value, Measurements, Recurring | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	831.9	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]
Debt Securities, Available for Sale	2,009.0	1,820.0
Fair Value, Measurements, Recurring | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	1,855.8	2,291.3
Fair Value, Measurements, Recurring | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2.8	4.3
Fair Value, Measurements, Recurring | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	427.7	435.1
Fair Value, Measurements, Recurring | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	19.1	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	(1,775.8)	(1,860.0)
Net Amount	1,633.6	794.1
Fair Value, Measurements, Recurring | Other Assets | Foreign Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Net Amount	3,308.8	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	100.6	97.0
Fair Value, Measurements, Recurring | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	749.9	1,207.6
Derivative liabilities, subject to netting arrangements	(2,463.3)	(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	3,063.1	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	113.4	83.5
Fair Value, Measurements, Recurring | Other Liabilities | Other Financial Derivatives
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	36.7	30.4
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	5,638.5	5,700.3
Total Available for Sale and Trading Debt Securities	5,638.5	5,700.3
Fair Value, Measurements, Recurring | Level 1 | U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	5,638.5	5,700.3
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	29,543.2	28,037.5
Trading Account	0.7	0.5
Total Available for Sale and Trading Debt Securities	29,543.9	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]
Debt Securities, Available for Sale	737.8	746.4
Fair Value, Measurements, Recurring | Level 2 | Government Sponsored Agency
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	21,054.5	18,676.6
Fair Value, Measurements, Recurring | Level 2 | Non-U.S. Government
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	141.2	177.2
Fair Value, Measurements, Recurring | Level 2 | Corporate Debt
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2,466.2	2,993.0
Fair Value, Measurements, Recurring | Level 2 | Covered Bonds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	831.9	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]
Debt Securities, Available for Sale	2,009.0	1,820.0
Fair Value, Measurements, Recurring | Level 2 | Other Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	1,855.8	2,291.3
Fair Value, Measurements, Recurring | Level 2 | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	427.7	435.1
Fair Value, Measurements, Recurring | Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	19.1	22.3
Fair Value, Measurements, Recurring | Level 2 | Other Assets
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Assets	3,409.4	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	3,308.8	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	100.6	97.0
Fair Value, Measurements, Recurring | Level 2 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	3,177.1	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	3,063.1	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	113.4	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	0.6	0.7
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2.8	4.3
Total Available for Sale and Trading Debt Securities	2.8	4.3
Fair Value, Measurements, Recurring | Level 3 | Auction Rate
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Debt Securities, Available for Sale	2.8	4.3
Fair Value, Measurements, Recurring | Level 3 | Other Liabilities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Derivative Liabilities	36.1	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	$ 36.1	$ 29.7