Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
ASSETS
Cash and Due from Banks	$ 4,462.2	$ 4,459.2
Federal Reserve and Other Central Bank Deposits	42,274.6	33,886.0
Interest-Bearing Deposits with Banks	2,793.8	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,505.5	712.8
Debt Securities
Available-for-sale	39,968.9	38,876.3
Debt securities, held-to-maturity	15,241.9	12,284.5
Loans (excluding Leases)
Client Security Settlement Receivables	2,069.9	845.7
Deposits
Savings Certificates and Other Time	1,505.7	986.7
Non U.S. Offices Interest-Bearing	67,277.5	60,400.3
Federal Funds Purchased	714.7	552.9
Securities Sold Under Agreements to Repurchase	36.5	489.7
Other Borrowings	7,477.9	6,744.8
Senior Notes	3,667.3	2,573.0
Long-Term Debt
Floating Rate Capital Debt	277.7	277.7
Derivative Instruments
Assets	2,767.6	2,844.0
Liabilities	2,619.8	2,312.4
LEVEL 2
Debt Securities
Available-for-sale	35,800.0	34,300.0
BOOK VALUE
ASSETS
Cash and Due from Banks	4,462.2	4,459.2
Federal Reserve and Other Central Bank Deposits	42,274.6	33,886.0
Interest-Bearing Deposits with Banks	2,793.8	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,505.5	712.8
Debt Securities
Available-for-sale	39,968.9	38,876.3
Debt securities, held-to-maturity	15,241.9	12,284.5
Trading Account	1.0	0.3
Loans (excluding Leases)
Held for Investment	33,496.1	31,239.5
Client Security Settlement Receivables	2,069.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	314.4	301.2
Community Development Investments	725.8	749.3
Employee Benefit and Deferred Compensation	208.1	199.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	53,270.9	47,733.6
Savings Certificates and Other Time	1,505.7	986.7
Non U.S. Offices Interest-Bearing	67,277.5	60,400.3
Federal Funds Purchased	714.7	552.9
Securities Sold Under Agreements to Repurchase	36.5	489.7
Other Borrowings	7,477.9	6,744.8
Senior Notes	3,667.3	2,573.0
Long-Term Debt
Subordinated Debt	1,200.4	1,148.1
Floating Rate Capital Debt	277.7	277.7
Other Liabilities
Standby Letters of Credit	26.8	25.5
Loan Commitments	57.7	32.3
BOOK VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	91.1	83.1
Liabilities	16.8	24.1
BOOK VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.1	20.5
Liabilities	15.6	21.1
BOOK VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	31.3	33.4
BOOK VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,881.2	3,151.7
Liabilities	2,823.1	3,158.1
BOOK VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	347.2	132.4
Liabilities	139.0	76.3
TOTAL FAIR VALUE
ASSETS
Cash and Due from Banks	4,462.2	4,459.2
Federal Reserve and Other Central Bank Deposits	42,274.6	33,886.0
Interest-Bearing Deposits with Banks	2,793.8	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,505.5	712.8
Debt Securities
Available-for-sale	39,968.9	38,876.3
Debt securities, held-to-maturity	15,223.4	12,249.3
Trading Account	1.0	0.3
Loans (excluding Leases)
Held for Investment	33,969.4	31,517.8
Client Security Settlement Receivables	2,069.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	314.4	301.2
Community Development Investments	725.8	749.3
Employee Benefit and Deferred Compensation	225.1	207.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	53,270.9	47,733.6
Savings Certificates and Other Time	1,518.9	994.2
Non U.S. Offices Interest-Bearing	67,277.5	60,400.3
Federal Funds Purchased	714.7	552.9
Securities Sold Under Agreements to Repurchase	36.5	489.7
Other Borrowings	7,480.3	6,745.9
Senior Notes	3,721.6	2,593.0
Long-Term Debt
Subordinated Debt	1,240.8	1,169.5
Floating Rate Capital Debt	256.0	262.1
Other Liabilities
Standby Letters of Credit	26.8	25.5
Loan Commitments	57.7	32.3
TOTAL FAIR VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	91.1	83.1
Liabilities	16.8	24.1
TOTAL FAIR VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.1	20.5
Liabilities	15.6	21.1
TOTAL FAIR VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	31.3	33.4
TOTAL FAIR VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,881.2	3,151.7
Liabilities	2,823.1	3,158.1
TOTAL FAIR VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	347.2	132.4
Liabilities	139.0	76.3
TOTAL FAIR VALUE | LEVEL 1
ASSETS
Cash and Due from Banks	4,462.2	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	4,176.4	4,549.1
Debt securities, held-to-maturity	98.0	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	145.1	131.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	53,270.9	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	42,274.6	33,886.0
Interest-Bearing Deposits with Banks	2,793.8	4,877.1
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,505.5	712.8
Debt Securities
Available-for-sale	35,792.5	34,327.2
Debt securities, held-to-maturity	15,125.4	12,110.5
Trading Account	1.0	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	2,069.9	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	314.4	301.2
Community Development Investments	725.8	749.3
Employee Benefit and Deferred Compensation	80.0	76.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	1,518.9	994.2
Non U.S. Offices Interest-Bearing	67,277.5	60,400.3
Federal Funds Purchased	714.7	552.9
Securities Sold Under Agreements to Repurchase	36.5	489.7
Other Borrowings	7,480.3	6,745.9
Senior Notes	3,721.6	2,593.0
Long-Term Debt
Subordinated Debt	1,240.8	1,169.5
Floating Rate Capital Debt	256.0	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	91.1	83.1
Liabilities	16.8	24.1
TOTAL FAIR VALUE | LEVEL 2 | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.1	20.5
Liabilities	15.6	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	2,881.2	3,151.7
Liabilities	2,823.1	3,158.1
TOTAL FAIR VALUE | LEVEL 2 | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	347.2	132.4
Liabilities	139.0	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	33,969.4	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	26.8	25.5
Loan Commitments	57.7	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	31.3	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