Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets
Cash and Due from Banks	$ 5,037.6	$ 4,581.6
Federal Reserve and Other Central Bank Deposits	22,493.7	30,080.2
Interest-Bearing Deposits with Banks	3,969.4	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	535.0	1,165.2
Debt Securities
Available for Sale	38,920.2	36,888.8
Held to Maturity	11,487.8	14,354.0
Loans (excluding Leases)
Client Security Settlement Receivables	2,102.2	1,646.1
Deposits
Savings Certificates and Other Time	895.2	688.7
Non U.S. Offices Interest-Bearing	55,694.8	66,468.0
Federal Funds Purchased	200.5	2,594.2
Securities Sold Under Agreements to Repurchase	265.7	168.3
Other Borrowings	7,799.6	7,901.7
Senior Notes	2,597.5	2,011.3
Long-Term Debt
Floating Rate Capital Debt	277.7	277.6
Derivative Instruments
Fair value asset	2,011.7	1,999.7
Fair value liability	1,849.6	1,947.0
Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	37.7	32.8
Level 2
Debt Securities
Available for Sale	33,300.0	31,700.0
Book Value
Assets
Cash and Due from Banks	5,037.6	4,581.6
Federal Reserve and Other Central Bank Deposits	22,493.7	30,080.2
Interest-Bearing Deposits with Banks	3,969.4	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	535.0	1,165.2
Debt Securities
Available for Sale	38,920.2	36,888.8
Held to Maturity	11,487.8	14,354.0
Trading Account	1.3	0.3
Loans (excluding Leases)
Held for Investment	30,651.0	32,287.0
Client Security Settlement Receivables	2,102.2	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	650.4	606.6
Employee Benefit and Deferred Compensation	198.8	202.3
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,674.2	37,340.1
Savings Certificates and Other Time	895.2	688.7
Non U.S. Offices Interest-Bearing	55,694.8	66,468.0
Federal Funds Purchased	200.5	2,594.2
Securities Sold Under Agreements to Repurchase	265.7	168.3
Other Borrowings	7,799.6	7,901.7
Senior Notes	2,597.5	2,011.3
Long-Term Debt
Subordinated Debt	1,159.7	1,112.4
Floating Rate Capital Debt	277.7	277.6
Other Liabilities
Standby Letters of Credit	27.6	30.8
Loan Commitments	33.3	34.3
Book Value | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	187.6	306.7
Fair value liability	22.0	72.5
Book Value | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	17.8	30.0
Fair value liability	20.7	24.5
Book Value | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	37.7	32.8
Book Value | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	2,101.4	2,159.4
Fair value liability	2,060.5	2,190.0
Book Value | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	159.8	66.1
Fair value liability	85.7	68.6
Total Fair Value
Assets
Cash and Due from Banks	5,037.6	4,581.6
Federal Reserve and Other Central Bank Deposits	22,493.7	30,080.2
Interest-Bearing Deposits with Banks	3,969.4	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	535.0	1,165.2
Debt Securities
Available for Sale	38,920.2	36,888.8
Held to Maturity	11,471.4	14,267.0
Trading Account	1.3	0.3
Loans (excluding Leases)
Held for Investment	30,932.5	32,339.2
Client Security Settlement Receivables	2,102.2	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	650.4	606.6
Employee Benefit and Deferred Compensation	212.1	194.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,674.2	37,340.1
Savings Certificates and Other Time	902.5	691.8
Non U.S. Offices Interest-Bearing	55,694.8	66,468.0
Federal Funds Purchased	200.5	2,594.2
Securities Sold Under Agreements to Repurchase	265.7	168.3
Other Borrowings	7,800.0	7,904.1
Senior Notes	2,602.7	1,994.4
Long-Term Debt
Subordinated Debt	1,176.4	1,089.7
Floating Rate Capital Debt	261.3	253.5
Other Liabilities
Standby Letters of Credit	27.6	30.8
Loan Commitments	33.3	34.3
Total Fair Value | Asset and Liability Management | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	187.6	306.7
Fair value liability	22.0	72.5
Total Fair Value | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	17.8	30.0
Fair value liability	20.7	24.5
Total Fair Value | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	37.7	32.8
Total Fair Value | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	2,101.4	2,159.4
Fair value liability	2,060.5	2,190.0
Total Fair Value | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	159.8	66.1
Fair value liability	85.7	68.6
Total Fair Value | Level 1
Assets
Cash and Due from Banks	5,037.6	4,581.6
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	5,653.0	5,185.3
Held to Maturity	113.8	101.6
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	138.5	125.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,674.2	37,340.1
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 asset		0.0
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	22,493.7	30,080.2
Interest-Bearing Deposits with Banks	3,969.4	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	535.0	1,165.2
Debt Securities
Available for Sale	33,267.2	31,703.5
Held to Maturity	11,357.6	14,165.4
Trading Account	1.3	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	2,102.2	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	650.4	606.6
Employee Benefit and Deferred Compensation	73.6	69.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	902.5	691.8
Non U.S. Offices Interest-Bearing	55,694.8	66,468.0
Federal Funds Purchased	200.5	2,594.2
Securities Sold Under Agreements to Repurchase	265.7	168.3
Other Borrowings	7,800.0	7,904.1
Senior Notes	2,602.7	1,994.4
Long-Term Debt
Subordinated Debt	1,176.4	1,089.7
Floating Rate Capital Debt	261.3	253.5
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	187.6	306.7
Fair value liability	22.0	72.5
Total Fair Value | Level 2 | Asset and Liability Management | Interest Rate Contracts
Derivative Instruments
Fair value asset	17.8	30.0
Fair value liability	20.7	24.5
Total Fair Value | Level 2 | Asset and Liability Management | Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	0.0	0.0
Total Fair Value | Level 2 | Client Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Fair value asset	2,101.4	2,159.4
Fair value liability	2,060.5	2,190.0
Total Fair Value | Level 2 | Client Related and Trading | Interest Rate Contracts
Derivative Instruments
Fair value asset	159.8	66.1
Fair value liability	85.7	68.6
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	0.0	0.0
Held to Maturity	0.0	0.0
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	30,932.5	32,339.2
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	27.6	30.8
Loan Commitments	33.3	34.3
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 asset		0.0
Fair value liability	37.7	32.8
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