Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
ASSETS
Cash and Due from Banks	$ 5,833.2	$ 4,459.2
Federal Reserve and Other Central Bank Deposits	44,661.5	33,886.0
Interest-Bearing Deposits with Banks	5,358.9	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	998.2	712.8
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	39,441.7	38,876.3
Held to Maturity	12,635.7	12,284.5
Loans (excluding Leases)
Client Security Settlement Receivables	2,116.9	845.7
Deposits
Savings Certificates and Other Time	918.7	986.7
Non U.S. Offices Interest-Bearing	69,779.1	60,400.3
Federal Funds Purchased	4,370.6	552.9
Securities Sold Under Agreements to Repurchase	34.1	489.7
Other Borrowings	5,955.9	6,744.8
Senior Notes	2,668.5	2,573.0
Long-Term Debt
Floating Rate Capital Debt	277.7	277.7
Derivative Instruments
Fair value asset	4,884.1	2,844.0
Fair value liability	5,187.4	2,312.4
LEVEL 2
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	34,900.0	34,300.0
Book Value
ASSETS
Cash and Due from Banks	5,833.2	4,459.2
Federal Reserve and Other Central Bank Deposits	44,661.5	33,886.0
Interest-Bearing Deposits with Banks	5,358.9	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	998.2	712.8
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	39,441.7	38,876.3
Held to Maturity	12,635.7	12,284.5
Trading Account	1.2	0.3
Loans (excluding Leases)
Held for Investment	37,626.2	31,239.5
Client Security Settlement Receivables	2,116.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	301.2
Community Development Investments	746.2	749.3
Employee Benefit and Deferred Compensation	204.7	199.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	60,793.4	47,733.6
Savings Certificates and Other Time	918.7	986.7
Non U.S. Offices Interest-Bearing	69,779.1	60,400.3
Federal Funds Purchased	4,370.6	552.9
Securities Sold Under Agreements to Repurchase	34.1	489.7
Other Borrowings	5,955.9	6,744.8
Senior Notes	2,668.5	2,573.0
Long-Term Debt
Subordinated Debt	1,196.0	1,148.1
Floating Rate Capital Debt	277.7	277.7
Other Liabilities
Standby Letters of Credit	27.2	25.5
Loan Commitments	49.0	32.3
Book Value | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	149.3	83.1
Fair value liability	17.3	24.1
Book Value | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	12.9	20.5
Fair value liability	18.3	21.1
Book Value | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value liability	26.5	33.4
Book Value | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	5,786.1	3,151.7
Fair value liability	5,746.1	3,158.1
Book Value | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	328.6	132.4
Fair value liability	141.8	76.3
Total Fair Value
ASSETS
Cash and Due from Banks	5,833.2	4,459.2
Federal Reserve and Other Central Bank Deposits	44,661.5	33,886.0
Interest-Bearing Deposits with Banks	5,358.9	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	998.2	712.8
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	39,441.7	38,876.3
Held to Maturity	12,533.0	12,249.3
Trading Account	1.2	0.3
Loans (excluding Leases)
Held for Investment	38,076.3	31,517.8
Client Security Settlement Receivables	2,116.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	301.2
Community Development Investments	746.2	749.3
Employee Benefit and Deferred Compensation	218.9	207.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	60,793.4	47,733.6
Savings Certificates and Other Time	927.9	994.2
Non U.S. Offices Interest-Bearing	69,779.1	60,400.3
Federal Funds Purchased	4,370.6	552.9
Securities Sold Under Agreements to Repurchase	34.1	489.7
Other Borrowings	5,949.8	6,745.9
Senior Notes	2,583.6	2,593.0
Long-Term Debt
Subordinated Debt	1,122.6	1,169.5
Floating Rate Capital Debt	255.5	262.1
Other Liabilities
Standby Letters of Credit	27.2	25.5
Loan Commitments	49.0	32.3
Total Fair Value | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	149.3	83.1
Fair value liability	17.3	24.1
Total Fair Value | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	12.9	20.5
Fair value liability	18.3	21.1
Total Fair Value | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value liability	26.5	33.4
Total Fair Value | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	5,786.1	3,151.7
Fair value liability	5,746.1	3,158.1
Total Fair Value | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	328.6	132.4
Fair value liability	141.8	76.3
Total Fair Value | LEVEL 1
ASSETS
Cash and Due from Banks	5,833.2	4,459.2
Federal Reserve and Other Central Bank Deposits	0.0	0.0
Interest-Bearing Deposits with Banks	0.0	0.0
Federal Funds Sold and Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	4,584.1	4,549.1
Held to Maturity	105.0	138.8
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	0.0	0.0
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	0.0	0.0
Community Development Investments	0.0	0.0
Employee Benefit and Deferred Compensation	147.1	131.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	60,793.4	47,733.6
Savings Certificates and Other Time	0.0	0.0
Non U.S. Offices Interest-Bearing	0.0	0.0
Federal Funds Purchased	0.0	0.0
Securities Sold Under Agreements to Repurchase	0.0	0.0
Other Borrowings	0.0	0.0
Senior Notes	0.0	0.0
Long-Term Debt
Subordinated Debt	0.0	0.0
Floating Rate Capital Debt	0.0	0.0
Other Liabilities
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Total Fair Value | LEVEL 1 | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 1 | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 1 | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 1 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 1 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 2
ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	44,661.5	33,886.0
Interest-Bearing Deposits with Banks	5,358.9	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	998.2	712.8
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	34,857.6	34,327.2
Held to Maturity	12,428.0	12,110.5
Trading Account	1.2	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	2,116.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	301.2
Community Development Investments	746.2	749.3
Employee Benefit and Deferred Compensation	71.8	76.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	927.9	994.2
Non U.S. Offices Interest-Bearing	69,779.1	60,400.3
Federal Funds Purchased	4,370.6	552.9
Securities Sold Under Agreements to Repurchase	34.1	489.7
Other Borrowings	5,949.8	6,745.9
Senior Notes	2,583.6	2,593.0
Long-Term Debt
Subordinated Debt	1,122.6	1,169.5
Floating Rate Capital Debt	255.5	262.1
Other Liabilities
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Total Fair Value | LEVEL 2 | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	149.3	83.1
Fair value liability	17.3	24.1
Total Fair Value | LEVEL 2 | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	12.9	20.5
Fair value liability	18.3	21.1
Total Fair Value | LEVEL 2 | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 2 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	5,786.1	3,151.7
Fair value liability	5,746.1	3,158.1
Total Fair Value | LEVEL 2 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	328.6	132.4
Fair value liability	141.8	76.3
Total Fair Value | LEVEL 3
ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	0.0	0.0
Interest-Bearing Deposits with Banks	0.0	0.0
Federal Funds Sold and Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities
Available for Sale (Amortized cost of $39,025.3 and $38,722.2)	0.0	0.0
Held to Maturity	0.0	0.0
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	38,076.3	31,517.8
Client Security Settlement Receivables	0.0	0.0
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	0.0	0.0
Community Development Investments	0.0	0.0
Employee Benefit and Deferred Compensation	0.0	0.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	0.0	0.0
Non U.S. Offices Interest-Bearing	0.0	0.0
Federal Funds Purchased	0.0	0.0
Securities Sold Under Agreements to Repurchase	0.0	0.0
Other Borrowings	0.0	0.0
Senior Notes	0.0	0.0
Long-Term Debt
Subordinated Debt	0.0	0.0
Floating Rate Capital Debt	0.0	0.0
Other Liabilities
Standby Letters of Credit	27.2	25.5
Loan Commitments	49.0	32.3
Total Fair Value | LEVEL 3 | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 3 | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 3 | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value liability	26.5	33.4
Total Fair Value | LEVEL 3 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value | LEVEL 3 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	$ 0.0	$ 0.0