Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
ASSETS
Cash and Due from Banks	$ 4,737.5	$ 4,459.2
Federal Reserve and Other Central Bank Deposits	39,244.6	33,886.0
Interest-Bearing Deposits with Banks	2,754.4	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,533.4	712.8
Debt Securities
Available-for-sale	42,551.0	38,876.3
Debt securities, held-to-maturity	16,754.0	12,284.5
Loans (excluding Leases)
Client Security Settlement Receivables	2,296.8	845.7
Deposits
Savings Certificates and Other Time	1,486.7	986.7
Non U.S. Offices Interest-Bearing	61,345.6	60,400.3
Federal Funds Purchased	2,203.7	552.9
Securities Sold Under Agreements to Repurchase	269.8	489.7
Other Borrowings	5,771.7	6,744.8
Senior Notes	3,655.6	2,573.0
Long-Term Debt
Floating Rate Capital Debt	277.7	277.7
Derivative Instruments
Assets	2,098.0	2,844.0
Liabilities	2,143.8	2,312.4
LEVEL 2
Debt Securities
Available-for-sale	38,400.0	34,300.0
BOOK VALUE
ASSETS
Cash and Due from Banks	4,737.5	4,459.2
Federal Reserve and Other Central Bank Deposits	39,244.6	33,886.0
Interest-Bearing Deposits with Banks	2,754.4	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,533.4	712.8
Debt Securities
Available-for-sale	42,551.0	38,876.3
Debt securities, held-to-maturity	16,754.0	12,284.5
Trading Account	0.6	0.3
Loans (excluding Leases)
Held for Investment	32,521.2	31,239.5
Client Security Settlement Receivables	2,296.8	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	239.7	301.2
Community Development Investments	761.3	749.3
Employee Benefit and Deferred Compensation	214.6	199.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	59,756.8	47,733.6
Savings Certificates and Other Time	1,486.7	986.7
Non U.S. Offices Interest-Bearing	61,345.6	60,400.3
Federal Funds Purchased	2,203.7	552.9
Securities Sold Under Agreements to Repurchase	269.8	489.7
Other Borrowings	5,771.7	6,744.8
Senior Notes	3,655.6	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	23.9	25.5
Loan Commitments	55.7	32.3
BOOK VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	36.8	83.1
Liabilities	112.8	24.1
BOOK VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.5	20.5
Liabilities	18.9	21.1
BOOK VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	32.3	33.4
BOOK VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,242.7	3,151.7
Liabilities	2,213.5	3,158.1
BOOK VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	323.2	132.4
Liabilities	123.3	76.3
TOTAL FAIR VALUE
ASSETS
Cash and Due from Banks	4,737.5	4,459.2
Federal Reserve and Other Central Bank Deposits	39,244.6	33,886.0
Interest-Bearing Deposits with Banks	2,754.4	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,533.4	712.8
Debt Securities
Available-for-sale	42,551.0	38,876.3
Debt securities, held-to-maturity	16,746.5	12,249.3
Trading Account	0.6	0.3
Loans (excluding Leases)
Held for Investment	33,001.4	31,517.8
Client Security Settlement Receivables	2,296.8	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	239.7	301.2
Community Development Investments	761.3	749.3
Employee Benefit and Deferred Compensation	230.0	207.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	59,756.8	47,733.6
Savings Certificates and Other Time	1,496.5	994.2
Non U.S. Offices Interest-Bearing	61,345.6	60,400.3
Federal Funds Purchased	2,203.7	552.9
Securities Sold Under Agreements to Repurchase	269.8	489.7
Other Borrowings	5,772.9	6,745.9
Senior Notes	3,731.0	2,593.0
Long-Term Debt
Subordinated Debt	1,250.0	1,169.5
Floating Rate Capital Debt	255.6	262.1
Other Liabilities
Standby Letters of Credit	23.9	25.5
Loan Commitments	55.7	32.3
TOTAL FAIR VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	36.8	83.1
Liabilities	112.8	24.1
TOTAL FAIR VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.5	20.5
Liabilities	18.9	21.1
TOTAL FAIR VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	32.3	33.4
TOTAL FAIR VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,242.7	3,151.7
Liabilities	2,213.5	3,158.1
TOTAL FAIR VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	323.2	132.4
Liabilities	123.3	76.3
TOTAL FAIR VALUE | LEVEL 1
ASSETS
Cash and Due from Banks	4,737.5	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	4,165.2	4,549.1
Debt securities, held-to-maturity	99.0	138.8
Trading Account	0.3	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	143.6	131.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	59,756.8	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/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 1 | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 1 | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 1 | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 1 | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	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	39,244.6	33,886.0
Interest-Bearing Deposits with Banks	2,754.4	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,533.4	712.8
Debt Securities
Available-for-sale	38,385.8	34,327.2
Debt securities, held-to-maturity	16,647.5	12,110.5
Trading Account	0.3	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	2,296.8	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	239.7	301.2
Community Development Investments	761.3	749.3
Employee Benefit and Deferred Compensation	86.4	76.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	1,496.5	994.2
Non U.S. Offices Interest-Bearing	61,345.6	60,400.3
Federal Funds Purchased	2,203.7	552.9
Securities Sold Under Agreements to Repurchase	269.8	489.7
Other Borrowings	5,772.9	6,745.9
Senior Notes	3,731.0	2,593.0
Long-Term Debt
Subordinated Debt	1,250.0	1,169.5
Floating Rate Capital Debt	255.6	262.1
Other Liabilities
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
TOTAL FAIR VALUE | LEVEL 2 | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	36.8	83.1
Liabilities	112.8	24.1
TOTAL FAIR VALUE | LEVEL 2 | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.5	20.5
Liabilities	18.9	21.1
TOTAL FAIR VALUE | LEVEL 2 | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 2 | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,242.7	3,151.7
Liabilities	2,213.5	3,158.1
TOTAL FAIR VALUE | LEVEL 2 | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	323.2	132.4
Liabilities	123.3	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	0.0	0.0
Debt securities, held-to-maturity	0.0	0.0
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	33,001.4	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	23.9	25.5
Loan Commitments	55.7	32.3
TOTAL FAIR VALUE | LEVEL 3 | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 3 | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 3 | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	32.3	33.4
TOTAL FAIR VALUE | LEVEL 3 | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	0.0	0.0
TOTAL FAIR VALUE | LEVEL 3 | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	0.0	0.0
Liabilities	$ 0.0	$ 0.0