Fair Value Measurements - Fair Value Of Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
ASSETS
Cash and Due from Banks	$ 4,459.2	$ 4,581.6
Federal Reserve and Other Central Bank Deposits	33,886.0	30,080.2
Interest-Bearing Deposits with Banks	4,877.1	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	712.8	1,165.2
Debt Securities
Available for Sale	38,876.3	36,888.8
Held to Maturity	12,284.5	14,354.0
Loans (excluding Leases)
Client Security Settlement Receivables	845.7	1,646.1
Deposits
Savings Certificates and Other Time	986.7	688.7
Non U.S. Offices Interest-Bearing	60,400.3	66,468.0
Federal Funds Purchased	552.9	2,594.2
Securities Sold Under Agreements to Repurchase	489.7	168.3
Other Borrowings	6,744.8	7,901.7
Senior Notes	2,573.0	2,011.3
Long-Term Debt
Floating Rate Capital Debt	277.7	277.6
DERIVATIVE INSTRUMENTS
Assets	2,844.0	1,999.7
Liabilities	2,312.4	1,947.0
Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		1.3
LEVEL 2
Debt Securities
Available for Sale	34,300.0	31,700.0
BOOK VALUE
ASSETS
Cash and Due from Banks	4,459.2	4,581.6
Federal Reserve and Other Central Bank Deposits	33,886.0	30,080.2
Interest-Bearing Deposits with Banks	4,877.1	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	712.8	1,165.2
Debt Securities
Available for Sale	38,876.3	36,888.8
Held to Maturity	12,284.5	14,354.0
Trading Account	0.3	0.3
Loans (excluding Leases)
Held for Investment	31,239.5	32,287.0
Client Security Settlement Receivables	845.7	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	749.3	606.6
Employee Benefit and Deferred Compensation	199.5	202.3
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	47,733.6	37,340.1
Savings Certificates and Other Time	986.7	688.7
Non U.S. Offices Interest-Bearing	60,400.3	66,468.0
Federal Funds Purchased	552.9	2,594.2
Securities Sold Under Agreements to Repurchase	489.7	168.3
Other Borrowings	6,744.8	7,901.7
Senior Notes	2,573.0	2,011.3
Long-Term Debt
Subordinated Debt	1,148.1	1,112.4
Floating Rate Capital Debt	277.7	277.6
Floating Rate Capital Debt
Standby Letters of Credit	25.5	30.8
Loan Commitments	32.3	34.3
BOOK VALUE | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	83.1	306.7
Liabilities	24.1	72.5
BOOK VALUE | Asset/Liability Management | Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		1.3
Liabilities	33.4	32.8
BOOK VALUE | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	20.5	30.0
Liabilities	21.1	24.5
BOOK VALUE | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	3,151.7	2,159.4
Liabilities	3,158.1	2,190.0
BOOK VALUE | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	132.4	66.1
Liabilities	76.3	68.6
Estimate of Fair Value, Fair Value Disclosure
ASSETS
Cash and Due from Banks	4,459.2	4,581.6
Federal Reserve and Other Central Bank Deposits	33,886.0	30,080.2
Interest-Bearing Deposits with Banks	4,877.1	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	712.8	1,165.2
Debt Securities
Available for Sale	38,876.3	36,888.8
Held to Maturity	12,249.3	14,267.0
Trading Account	0.3	0.3
Loans (excluding Leases)
Held for Investment	31,517.8	32,339.2
Client Security Settlement Receivables	845.7	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	749.3	606.6
Employee Benefit and Deferred Compensation	207.6	194.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	47,733.6	37,340.1
Savings Certificates and Other Time	994.2	691.8
Non U.S. Offices Interest-Bearing	60,400.3	66,468.0
Federal Funds Purchased	552.9	2,594.2
Securities Sold Under Agreements to Repurchase	489.7	168.3
Other Borrowings	6,745.9	7,904.1
Senior Notes	2,593.0	1,994.4
Long-Term Debt
Subordinated Debt	1,169.5	1,089.7
Floating Rate Capital Debt	262.1	253.5
Floating Rate Capital Debt
Standby Letters of Credit	25.5	30.8
Loan Commitments	32.3	34.3
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	83.1	306.7
Liabilities	24.1	72.5
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		1.3
Liabilities	33.4	32.8
Estimate of Fair Value, Fair Value Disclosure | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	20.5	30.0
Liabilities	21.1	24.5
Estimate of Fair Value, Fair Value Disclosure | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	3,151.7	2,159.4
Liabilities	3,158.1	2,190.0
Estimate of Fair Value, Fair Value Disclosure | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	132.4	66.1
Liabilities	76.3	68.6
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1
ASSETS
Cash and Due from Banks	4,459.2	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	4,549.1	5,185.3
Held to Maturity	138.8	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	131.0	125.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	47,733.6	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
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
Liabilities	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 1 | Asset/Liability Management | Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		0.0
Liabilities	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
Liabilities	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
Liabilities	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
Liabilities	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	33,886.0	30,080.2
Interest-Bearing Deposits with Banks	4,877.1	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	712.8	1,165.2
Debt Securities
Available for Sale	34,327.2	31,703.5
Held to Maturity	12,110.5	14,165.4
Trading Account	0.3	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	845.7	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	749.3	606.6
Employee Benefit and Deferred Compensation	76.6	69.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	994.2	691.8
Non U.S. Offices Interest-Bearing	60,400.3	66,468.0
Federal Funds Purchased	552.9	2,594.2
Securities Sold Under Agreements to Repurchase	489.7	168.3
Other Borrowings	6,745.9	7,904.1
Senior Notes	2,593.0	1,994.4
Long-Term Debt
Subordinated Debt	1,169.5	1,089.7
Floating Rate Capital Debt	262.1	253.5
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	83.1	306.7
Liabilities	24.1	72.5
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Asset/Liability Management | Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		1.3
Liabilities	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	20.5	30.0
Liabilities	21.1	24.5
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	3,151.7	2,159.4
Liabilities	3,158.1	2,190.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 2 | Client-Related and Trading | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	132.4	66.1
Liabilities	76.3	68.6
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 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	31,517.8	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
Floating Rate Capital Debt
Standby Letters of Credit	25.5	30.8
Loan Commitments	32.3	34.3
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Liabilities	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Interest Rate Swaps Exchange Cleared
DERIVATIVE INSTRUMENTS
Assets		0.0
Liabilities	33.4	32.8
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Asset/Liability Management | Interest Rate Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Liabilities	0.0	0.0
Estimate of Fair Value, Fair Value Disclosure | LEVEL 3 | Client-Related and Trading | Foreign Exchange Contracts
DERIVATIVE INSTRUMENTS
Assets	0.0	0.0
Liabilities	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
Liabilities	$ 0.0	$ 0.0