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Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Assets
Cash and Due from Banks	$ 5,348.3	$ 4,581.6	$ 4,888.1	$ 4,518.1
Federal Reserve and Other Central Bank Deposits	27,553.5	30,080.2
Interest-Bearing Deposits with Banks	4,193.6	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	660.5	1,165.2
Debt Securities
Available for Sale	37,867.5	36,888.8
Held to Maturity	10,351.2	14,354.0
Loans (excluding Leases)
Client Security Settlement Receivables	1,311.1	1,646.1
Deposits
Savings Certificates and Other Time	897.3	688.7
Non U.S. Offices Interest-Bearing	59,028.0	66,468.0
Federal Funds Purchased	368.0	2,594.2
Securities Sold Under Agreements to Repurchase	125.2	168.3
Other Borrowings	7,766.4	7,901.7
Senior Notes	2,565.6	2,011.3
Long-Term Debt
Floating Rate Capital Debt	277.6	277.6
Derivative Instruments
Fair value asset	1,512.1	1,999.7
Fair value liability	1,416.4	1,947.0
Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	35.3	32.8
Level 2
Debt Securities
Available for Sale	32,800.0	31,700.0
Book Value
Assets
Cash and Due from Banks	5,348.3	4,581.6
Federal Reserve and Other Central Bank Deposits	27,553.5	30,080.2
Interest-Bearing Deposits with Banks	4,193.6	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	660.5	1,165.2
Debt Securities
Available for Sale	37,867.5	36,888.8
Held to Maturity	10,351.2	14,354.0
Trading Account	0.8	0.3
Loans (excluding Leases)
Held for Investment	30,783.1	32,287.0
Client Security Settlement Receivables	1,311.1	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	589.9	606.6
Employee Benefit and Deferred Compensation	203.8	202.3
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,305.1	37,340.1
Savings Certificates and Other Time	897.3	688.7
Non U.S. Offices Interest-Bearing	59,028.0	66,468.0
Federal Funds Purchased	368.0	2,594.2
Securities Sold Under Agreements to Repurchase	125.2	168.3
Other Borrowings	7,766.4	7,901.7
Senior Notes	2,565.6	2,011.3
Long-Term Debt
Subordinated Debt	1,147.6	1,112.4
Floating Rate Capital Debt	277.6	277.6
Other Liabilities
Standby Letters of Credit	30.5	30.8
Loan Commitments	32.4	34.3
Book Value \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	62.3	306.7
Fair value liability	18.1	72.5
Book Value \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Fair value asset	22.6	30.0
Fair value liability	24.6	24.5
Book Value \| Asset And Liability Management \| Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	35.3	32.8
Book Value \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	1,679.1	2,159.4
Fair value liability	1,664.6	2,190.0
Book Value \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Fair value asset	127.8	66.1
Fair value liability	72.9	68.6
Total Fair Value
Assets
Cash and Due from Banks	5,348.3	4,581.6
Federal Reserve and Other Central Bank Deposits	27,553.5	30,080.2
Interest-Bearing Deposits with Banks	4,193.6	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	660.5	1,165.2
Debt Securities
Available for Sale	37,867.5	36,888.8
Held to Maturity	10,325.4	14,267.0
Trading Account	0.8	0.3
Loans (excluding Leases)
Held for Investment	31,048.6	32,339.2
Client Security Settlement Receivables	1,311.1	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	589.9	606.6
Employee Benefit and Deferred Compensation	208.8	194.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,305.1	37,340.1
Savings Certificates and Other Time	904.9	691.8
Non U.S. Offices Interest-Bearing	59,028.0	66,468.0
Federal Funds Purchased	368.0	2,594.2
Securities Sold Under Agreements to Repurchase	125.2	168.3
Other Borrowings	7,772.2	7,904.1
Senior Notes	2,582.9	1,994.4
Long-Term Debt
Subordinated Debt	1,163.1	1,089.7
Floating Rate Capital Debt	257.1	253.5
Other Liabilities
Standby Letters of Credit	30.5	30.8
Loan Commitments	32.4	34.3
Total Fair Value \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	62.3	306.7
Fair value liability	18.1	72.5
Total Fair Value \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Fair value asset	22.6	30.0
Fair value liability	24.6	24.5
Total Fair Value \| Asset And Liability Management \| Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	35.3	32.8
Total Fair Value \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	1,679.1	2,159.4
Fair value liability	1,664.6	2,190.0
Total Fair Value \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Fair value asset	127.8	66.1
Fair value liability	72.9	68.6
Total Fair Value \| Level 1
Assets
Cash and Due from Banks	5,348.3	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	5,079.4	5,185.3
Held to Maturity	117.7	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	135.3	125.0
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	40,305.1	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
Other Liabilities
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Total Fair Value \| Level 1 \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 1 \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 1 \| Asset And Liability Management \| Other Financial Derivatives
Derivative Instruments
Fair value asset		0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 1 \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 1 \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	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	27,553.5	30,080.2
Interest-Bearing Deposits with Banks	4,193.6	4,264.2
Federal Funds Sold and Securities Purchased under Agreements to Resell	660.5	1,165.2
Debt Securities
Available for Sale	32,788.1	31,703.5
Held to Maturity	10,207.7	14,165.4
Trading Account	0.8	0.3
Loans (excluding Leases)
Held for Investment	0.0	0.0
Client Security Settlement Receivables	1,311.1	1,646.1
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	301.2	300.3
Community Development Investments	589.9	606.6
Employee Benefit and Deferred Compensation	73.5	69.5
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	904.9	691.8
Non U.S. Offices Interest-Bearing	59,028.0	66,468.0
Federal Funds Purchased	368.0	2,594.2
Securities Sold Under Agreements to Repurchase	125.2	168.3
Other Borrowings	7,772.2	7,904.1
Senior Notes	2,582.9	1,994.4
Long-Term Debt
Subordinated Debt	1,163.1	1,089.7
Floating Rate Capital Debt	257.1	253.5
Other Liabilities
Standby Letters of Credit	0.0	0.0
Loan Commitments	0.0	0.0
Total Fair Value \| Level 2 \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	62.3	306.7
Fair value liability	18.1	72.5
Total Fair Value \| Level 2 \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Fair value asset	22.6	30.0
Fair value liability	24.6	24.5
Total Fair Value \| Level 2 \| Asset And Liability Management \| Other Financial Derivatives
Derivative Instruments
Fair value asset		1.3
Fair value liability	0.6	0.0
Total Fair Value \| Level 2 \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	1,679.1	2,159.4
Fair value liability	1,664.6	2,190.0
Total Fair Value \| Level 2 \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Fair value asset	127.8	66.1
Fair value liability	72.9	68.6
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
Held to Maturity	0.0	0.0
Trading Account	0.0	0.0
Loans (excluding Leases)
Held for Investment	31,048.6	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
Other Liabilities
Standby Letters of Credit	30.5	30.8
Loan Commitments	32.4	34.3
Total Fair Value \| Level 3 \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 3 \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 3 \| Asset And Liability Management \| Other Financial Derivatives
Derivative Instruments
Fair value asset		0.0
Fair value liability	34.7	32.8
Total Fair Value \| Level 3 \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	0.0	0.0
Total Fair Value \| Level 3 \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Fair value asset	0.0	0.0
Fair value liability	$ 0.0	$ 0.0