Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
ASSETS
Cash and Due from Banks	$ 4,584.4	$ 4,389.5
Federal Reserve and Other Central Bank Deposits	46,911.4	55,503.6
Interest-Bearing Deposits with Banks	5,142.8	4,372.6
Securities Purchased under Agreements to Resell	1,109.3	1,596.5
Debt Securities
Available-for-sale	42,878.6	42,022.0
Debt securities, held-to-maturity	17,136.4	17,791.1
Loans (excluding Leases)
Client Security Settlement Receivables	1,970.2	1,160.2
Deposits
Savings Certificates and Other Time	909.7	937.1
Non U.S. Offices Interest-Bearing	63,746.5	71,198.4
Federal Funds Purchased	0.2	260.2
Securities Sold Under Agreements to Repurchase	95.6	39.8
Other Borrowings	5,428.4	4,011.5
Senior Notes	2,991.2	3,122.4
Long-Term Debt
Floating Rate Capital Debt	277.8	277.8
Derivative Instruments
Assets	2,509.4	4,097.2
Liabilities	2,251.7	3,738.0
LEVEL 2
Debt Securities
Available-for-sale	40,300.0	39,200.0
BOOK VALUE
ASSETS
Cash and Due from Banks	4,584.4	4,389.5
Federal Reserve and Other Central Bank Deposits	46,911.4	55,503.6
Interest-Bearing Deposits with Banks	5,142.8	4,372.6
Federal Funds Sold	0.1
Securities Purchased under Agreements to Resell	1,109.3	1,596.5
Debt Securities
Available-for-sale	42,878.6	42,022.0
Debt securities, held-to-maturity	17,136.4	17,791.1
Trading Account	0.4	0.5
Loans (excluding Leases)
Held for Investment	34,168.1	33,558.0
Client Security Settlement Receivables	1,970.2	1,160.2
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	230.0	275.0
Community Development Investments	908.1	919.6
Employee Benefit and Deferred Compensation	230.3	215.8
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	72,801.2	71,742.5
Savings Certificates and Other Time	909.7	937.1
Non U.S. Offices Interest-Bearing	63,746.5	71,198.4
Federal Funds Purchased	0.2	260.2
Securities Sold Under Agreements to Repurchase	95.6	39.8
Other Borrowings	5,428.4	4,011.5
Senior Notes	2,991.2	3,122.4
Long-Term Debt
Subordinated Debt	1,165.9	1,189.3
Floating Rate Capital Debt	277.8	277.8
Other Liabilities
Standby Letters of Credit	18.7	22.4
Loan Commitments	76.9	77.0
BOOK VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	370.9	15.6
Liabilities	29.3	311.8
BOOK VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.0	8.3
Liabilities	13.7	10.2
BOOK VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	34.6	35.3
BOOK VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,630.1	4,245.1
Liabilities	2,513.7	4,410.7
BOOK VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	195.6	289.2
Liabilities	110.0	114.8
TOTAL FAIR VALUE
ASSETS
Cash and Due from Banks	4,584.4	4,389.5
Federal Reserve and Other Central Bank Deposits	46,911.4	55,503.6
Interest-Bearing Deposits with Banks	5,142.8	4,372.6
Federal Funds Sold	0.1
Securities Purchased under Agreements to Resell	1,109.3	1,596.5
Debt Securities
Available-for-sale	42,878.6	42,022.0
Debt securities, held-to-maturity	17,100.0	17,797.4
Trading Account	0.4	0.5
Loans (excluding Leases)
Held for Investment	34,181.7	34,017.5
Client Security Settlement Receivables	1,970.2	1,160.2
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	230.0	275.0
Community Development Investments	908.1	919.6
Employee Benefit and Deferred Compensation	234.5	228.9
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	72,801.2	71,742.5
Savings Certificates and Other Time	911.0	943.0
Non U.S. Offices Interest-Bearing	63,746.5	71,198.4
Federal Funds Purchased	0.2	260.2
Securities Sold Under Agreements to Repurchase	95.6	39.8
Other Borrowings	5,428.6	4,012.7
Senior Notes	3,083.9	3,222.6
Long-Term Debt
Subordinated Debt	1,211.5	1,250.1
Floating Rate Capital Debt	268.7	264.6
Other Liabilities
Standby Letters of Credit	18.7	22.4
Loan Commitments	76.9	77.0
TOTAL FAIR VALUE | Asset/Liability Management | Foreign Exchange Contracts
Derivative Instruments
Assets	370.9	15.6
Liabilities	29.3	311.8
TOTAL FAIR VALUE | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.0	8.3
Liabilities	13.7	10.2
TOTAL FAIR VALUE | Asset/Liability Management | Other Financial Derivatives
Derivative Instruments
Liabilities	34.6	35.3
TOTAL FAIR VALUE | Client-Related and Trading | Foreign Exchange Contracts
Derivative Instruments
Assets	2,630.1	4,245.1
Liabilities	2,513.7	4,410.7
TOTAL FAIR VALUE | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	195.6	289.2
Liabilities	110.0	114.8
TOTAL FAIR VALUE | LEVEL 1
ASSETS
Cash and Due from Banks	4,584.4	4,389.5
Federal Reserve and Other Central Bank Deposits	0.0	0.0
Interest-Bearing Deposits with Banks	0.0	0.0
Federal Funds Sold	0.0
Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities
Available-for-sale	2,561.0	2,799.9
Debt securities, held-to-maturity	100.0	90.0
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	133.9	138.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	72,801.2	71,742.5
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	46,911.4	55,503.6
Interest-Bearing Deposits with Banks	5,142.8	4,372.6
Federal Funds Sold	0.1
Securities Purchased under Agreements to Resell	1,109.3	1,596.5
Debt Securities
Available-for-sale	40,317.6	39,222.1
Debt securities, held-to-maturity	17,000.0	17,707.4
Trading Account	0.4	0.5
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	1,970.2	1,160.2
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	230.0	275.0
Community Development Investments	908.1	919.6
Employee Benefit and Deferred Compensation	100.6	90.3
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	911.0	943.0
Non U.S. Offices Interest-Bearing	63,746.5	71,198.4
Federal Funds Purchased	0.2	260.2
Securities Sold Under Agreements to Repurchase	95.6	39.8
Other Borrowings	5,428.6	4,012.7
Senior Notes	3,083.9	3,222.6
Long-Term Debt
Subordinated Debt	1,211.5	1,250.1
Floating Rate Capital Debt	268.7	264.6
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	370.9	15.6
Liabilities	29.3	311.8
TOTAL FAIR VALUE | LEVEL 2 | Asset/Liability Management | Interest Rate Contracts
Derivative Instruments
Assets	10.0	8.3
Liabilities	13.7	10.2
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	2,630.1	4,245.1
Liabilities	2,513.7	4,410.7
TOTAL FAIR VALUE | LEVEL 2 | Client-Related and Trading | Interest Rate Contracts
Derivative Instruments
Assets	195.6	289.2
Liabilities	110.0	114.8
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	0.0
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	34,181.7	34,017.5
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	18.7	22.4
Loan Commitments	76.9	77.0
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	34.6	35.3
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