Fair Value Measurements - Fair Value Of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
ASSETS
Cash and Due from Banks	$ 4,389.5	$ 4,459.2
Federal Reserve and Other Central Bank Deposits	55,503.6	33,886.0
Interest-Bearing Deposits with Banks	4,372.6	4,877.1
Federal Funds Sold	1,600.0
Securities Purchased under Agreements to Resell	1,596.5	707.8
Debt Securities
Available for Sale	42,022.0	38,876.3
Held to Maturity	17,791.1	12,284.5
Loans (excluding Leases)
Client Security Settlement Receivables	1,160.2	845.7
Deposits
Savings Certificates and Other Time	937.1	986.7
Non U.S. Offices Interest-Bearing	71,198.4	60,400.3
Federal Funds Purchased	260.2	552.9
Securities Sold Under Agreements to Repurchase	39.8	489.7
Other Borrowings	4,011.5	6,744.8
Senior Notes	3,122.4	2,573.0
Long-Term Debt
Floating Rate Capital Debt	277.8	277.7
DERIVATIVE INSTRUMENTS
Assets	4,097.2	2,844.0
Derivative Liability, Fair Value, Gross Liability	3,738.0	2,312.4
LEVEL 2
Debt Securities
Available for Sale	39,200.0	34,300.0
BOOK VALUE
ASSETS
Cash and Due from Banks	4,389.5	4,459.2
Federal Reserve and Other Central Bank Deposits	55,503.6	33,886.0
Interest-Bearing Deposits with Banks	4,372.6	4,877.1
Federal Funds Sold		5.0
Securities Purchased under Agreements to Resell	1,596.5	707.8
Debt Securities
Available for Sale	42,022.0	38,876.3
Held to Maturity	17,791.1	12,284.5
Trading Account	0.5	0.3
Loans (excluding Leases)
Held for Investment	33,558.0	31,239.5
Client Security Settlement Receivables	1,160.2	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	275.0	301.2
Community Development Investments	919.6	749.3
Employee Benefit and Deferred Compensation	215.8	199.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	71,742.5	47,733.6
Savings Certificates and Other Time	937.1	986.7
Non U.S. Offices Interest-Bearing	71,198.4	60,400.3
Federal Funds Purchased	260.2	552.9
Securities Sold Under Agreements to Repurchase	39.8	489.7
Other Borrowings	4,011.5	6,744.8
Senior Notes	3,122.4	2,573.0
Long-Term Debt
Subordinated Debt	1,189.3	1,148.1
Floating Rate Capital Debt	277.8	277.7
Floating Rate Capital Debt
Standby Letters of Credit	22.4	25.5
Loan Commitments	77.0	32.3
BOOK VALUE | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	15.6	83.1
Derivative Liability, Fair Value, Gross Liability	311.8	24.1
BOOK VALUE | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	8.3	20.5
Derivative Liability, Fair Value, Gross Liability	10.2	21.1
BOOK VALUE | Asset/Liability Management | Other Financial Derivatives
DERIVATIVE INSTRUMENTS
Derivative Liability, Fair Value, Gross Liability	35.3	33.4
BOOK VALUE | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	4,245.1	3,151.7
Derivative Liability, Fair Value, Gross Liability	4,410.7	3,158.1
BOOK VALUE | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	289.2	132.4
Derivative Liability, Fair Value, Gross Liability	114.8	76.3
Estimate of Fair Value, Fair Value Disclosure
ASSETS
Cash and Due from Banks	4,389.5	4,459.2
Federal Reserve and Other Central Bank Deposits	55,503.6	33,886.0
Interest-Bearing Deposits with Banks	4,372.6	4,877.1
Federal Funds Sold		5.0
Securities Purchased under Agreements to Resell	1,596.5	707.8
Debt Securities
Available for Sale	42,022.0	38,876.3
Held to Maturity	17,797.4	12,249.3
Trading Account	0.5	0.3
Loans (excluding Leases)
Held for Investment	34,017.5	31,517.8
Client Security Settlement Receivables	1,160.2	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	275.0	301.2
Community Development Investments	919.6	749.3
Employee Benefit and Deferred Compensation	228.9	207.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	71,742.5	47,733.6
Savings Certificates and Other Time	943.0	994.2
Non U.S. Offices Interest-Bearing	71,198.4	60,400.3
Federal Funds Purchased	260.2	552.9
Securities Sold Under Agreements to Repurchase	39.8	489.7
Other Borrowings	4,012.7	6,745.9
Senior Notes	3,222.6	2,593.0
Long-Term Debt
Subordinated Debt	1,250.1	1,169.5
Floating Rate Capital Debt	264.6	262.1
Floating Rate Capital Debt
Standby Letters of Credit	22.4	25.5
Loan Commitments	77.0	32.3
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	15.6	83.1
Derivative Liability, Fair Value, Gross Liability	311.8	24.1
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	8.3	20.5
Derivative Liability, Fair Value, Gross Liability	10.2	21.1
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Other Financial Derivatives
DERIVATIVE INSTRUMENTS
Derivative Liability, Fair Value, Gross Liability	35.3	33.4
Estimate of Fair Value, Fair Value Disclosure | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	4,245.1	3,151.7
Derivative Liability, Fair Value, Gross Liability	4,410.7	3,158.1
Estimate of Fair Value, Fair Value Disclosure | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	289.2	132.4
Derivative Liability, Fair Value, Gross Liability	114.8	76.3
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1
ASSETS
Cash and Due from Banks	4,389.5	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		0.0
Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities
Available for Sale	2,799.9	4,549.1
Held to Maturity	90.0	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	138.6	131.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	71,742.5	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
Floating Rate Capital Debt
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Asset/Liability Management | Other Financial Derivatives
DERIVATIVE INSTRUMENTS
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2
ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	55,503.6	33,886.0
Interest-Bearing Deposits with Banks	4,372.6	4,877.1
Federal Funds Sold		5.0
Securities Purchased under Agreements to Resell	1,596.5	707.8
Debt Securities
Available for Sale	39,222.1	34,327.2
Held to Maturity	17,707.4	12,110.5
Trading Account	0.5	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	1,160.2	845.7
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	275.0	301.2
Community Development Investments	919.6	749.3
Employee Benefit and Deferred Compensation	90.3	76.6
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	943.0	994.2
Non U.S. Offices Interest-Bearing	71,198.4	60,400.3
Federal Funds Purchased	260.2	552.9
Securities Sold Under Agreements to Repurchase	39.8	489.7
Other Borrowings	4,012.7	6,745.9
Senior Notes	3,222.6	2,593.0
Long-Term Debt
Subordinated Debt	1,250.1	1,169.5
Floating Rate Capital Debt	264.6	262.1
Floating Rate Capital Debt
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	15.6	83.1
Derivative Liability, Fair Value, Gross Liability	311.8	24.1
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	8.3	20.5
Derivative Liability, Fair Value, Gross Liability	10.2	21.1
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Asset/Liability Management | Other Financial Derivatives
DERIVATIVE INSTRUMENTS
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	4,245.1	3,151.7
Derivative Liability, Fair Value, Gross Liability	4,410.7	3,158.1
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	289.2	132.4
Derivative Liability, Fair Value, Gross Liability	114.8	76.3
Estimate of Fair Value, Fair Value Disclosure | 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
Held to Maturity	0.0	0.0
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	34,017.5	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
Floating Rate Capital Debt
Standby Letters of Credit	22.4	25.5
Loan Commitments	77.0	32.3
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Other Financial Derivatives
DERIVATIVE INSTRUMENTS
Derivative Liability, Fair Value, Gross Liability	35.3	33.4
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Derivative Liability, Fair Value, Gross Liability	$ 0.0	$ 0.0