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Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Assets
Cash and Due from Banks	$ 4,485.9	$ 3,050.6	$ 2,918.9	$ 3,162.4
Federal Funds Sold and Resell Agreements	1,255.6	1,062.7
Interest-Bearing Deposits with Banks	13,935.0	14,928.3
Federal Reserve Deposits	22,252.0	17,386.3
Securities
Available for Sale	31,091.7	29,558.5
Held to Maturity	5,846.5	4,170.8
Trading Account	0.5	4.7
Loans (excluding Leases)
Held for Sale	335.3	2.5
Client Security Settlement Receivables	1,918.4	1,568.8
Deposits
Savings Certificates and Other Time	1,457.1	1,757.4
Non U.S. Offices Interest-Bearing	53,823.8	47,545.0
Federal Funds Purchased	343.5	932.9
Securities Sold Under Agreements to Repurchase	484.5	885.1
Other Borrowings	4,080.6	1,685.2
Senior Notes	1,497.3	1,497.0
Long Term Debt (excluding Leases)
Floating Rate Capital Debt	277.3	277.2
Asset And Liability Management
Derivative Instruments
Assets	182.6	251.7
Liabilities	49.5	48.7
Client Related and Trading
Derivative Instruments
Assets	2,918.1	4,255.0
Liabilities	2,888.0	4,173.3
Carrying (Reported) Amount, Fair Value Disclosure
Assets
Cash and Due from Banks	4,485.9	3,050.6
Federal Funds Sold and Resell Agreements	1,255.6	1,062.7
Interest-Bearing Deposits with Banks	13,935.0	14,928.3
Federal Reserve Deposits	22,252.0	17,386.3
Securities
Available for Sale	31,091.7	29,558.5
Held to Maturity	5,846.5	4,170.8
Trading Account	0.5	4.7
Loans (excluding Leases)
Held for Investment	31,997.3	30,458.0
Held for Sale	335.3	2.5
Client Security Settlement Receivables	1,918.4	1,568.8
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	253.1	207.5
Community Development Investments	176.5	209.9
Employee Benefit and Deferred Compensation	155.5	143.2
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	44,655.0	41,454.6
Savings Certificates and Other Time	1,457.1	1,757.4
Non U.S. Offices Interest-Bearing	53,823.8	47,545.0
Federal Funds Purchased	343.5	932.9
Securities Sold Under Agreements to Repurchase	484.5	885.1
Other Borrowings	4,080.6	1,685.2
Senior Notes	1,497.3	1,497.0
Long Term Debt (excluding Leases)
Subordinated Debt	1,363.2	1,583.3
Floating Rate Capital Debt	277.3	277.2
Other Liabilities
Standby Letters of Credit	38.1	60.1
Loan Commitments	24.6	28.3
Carrying (Reported) Amount, Fair Value Disclosure \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Assets	39.3	125.7
Liabilities	22.7	23.5
Carrying (Reported) Amount, Fair Value Disclosure \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Assets	143.7	126.8
Liabilities	29.4	30.5
Carrying (Reported) Amount, Fair Value Disclosure \| Asset And Liability Management \| Other Financial Derivatives (1)
Derivative Instruments
Liabilities	10.9
Carrying (Reported) Amount, Fair Value Disclosure \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Assets	2,779.0	4,149.5
Liabilities	2,751.9	4,072.0
Carrying (Reported) Amount, Fair Value Disclosure \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Assets	139.1	105.5
Liabilities	136.1	101.3
Estimate of Fair Value, Fair Value Disclosure
Assets
Cash and Due from Banks	4,485.9	3,050.6
Federal Funds Sold and Resell Agreements	1,255.6	1,062.7
Interest-Bearing Deposits with Banks	13,935.0	14,928.3
Federal Reserve Deposits	22,252.0	17,386.3
Securities
Available for Sale	31,091.7	29,558.5
Held to Maturity	5,832.6	4,176.1
Trading Account	0.5	4.7
Loans (excluding Leases)
Held for Investment	32,245.0	30,600.4
Held for Sale	335.3	2.5
Client Security Settlement Receivables	1,918.4	1,568.8
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	253.1	207.5
Community Development Investments	178.7	210.8
Employee Benefit and Deferred Compensation	154.3	146.7
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	44,655.0	41,454.6
Savings Certificates and Other Time	1,469.3	1,757.0
Non U.S. Offices Interest-Bearing	53,823.8	47,545.0
Federal Funds Purchased	343.5	932.9
Securities Sold Under Agreements to Repurchase	484.5	885.1
Other Borrowings	4,085.4	1,686.2
Senior Notes	1,544.5	1,541.8
Long Term Debt (excluding Leases)
Subordinated Debt	1,342.9	1,583.4
Floating Rate Capital Debt	238.4	242.8
Other Liabilities
Standby Letters of Credit	38.1	60.1
Loan Commitments	24.6	28.3
Estimate of Fair Value, Fair Value Disclosure \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Assets	39.3	125.7
Liabilities	22.7	23.5
Estimate of Fair Value, Fair Value Disclosure \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Assets	143.7	126.8
Liabilities	29.4	30.5
Estimate of Fair Value, Fair Value Disclosure \| Asset And Liability Management \| Other Financial Derivatives (1)
Derivative Instruments
Liabilities	10.9
Estimate of Fair Value, Fair Value Disclosure \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Assets	2,779.0	4,149.5
Liabilities	2,751.9	4,072.0
Estimate of Fair Value, Fair Value Disclosure \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Assets	139.1	105.5
Liabilities	136.1	101.3
Estimate of Fair Value, Fair Value Disclosure \| Level 1
Assets
Cash and Due from Banks	4,485.9	3,050.6
Securities
Available for Sale	5,125.1	4,506.9
Other Assets
Employee Benefit and Deferred Compensation	106.2	96.7
Deposits
Demand, Noninterest-Bearing, Savings and Money Market	44,655.0	41,454.6
Estimate of Fair Value, Fair Value Disclosure \| Level 2
Assets
Federal Funds Sold and Resell Agreements	1,255.6	1,062.7
Interest-Bearing Deposits with Banks	13,935.0	14,928.3
Federal Reserve Deposits	22,252.0	17,386.3
Securities
Available for Sale	25,950.4	25,033.5
Held to Maturity	5,832.6	4,176.1
Trading Account	0.5	4.7
Loans (excluding Leases)
Client Security Settlement Receivables	1,918.4	1,568.8
Other Assets
Federal Reserve and Federal Home Loan Bank Stock	253.1	207.5
Community Development Investments	178.7	210.8
Employee Benefit and Deferred Compensation	48.1	50.0
Deposits
Savings Certificates and Other Time	1,469.3	1,757.0
Non U.S. Offices Interest-Bearing	53,823.8	47,545.0
Federal Funds Purchased	343.5	932.9
Securities Sold Under Agreements to Repurchase	484.5	885.1
Other Borrowings	4,085.4	1,686.2
Senior Notes	1,544.5	1,541.8
Long Term Debt (excluding Leases)
Subordinated Debt	1,342.9	1,583.4
Floating Rate Capital Debt	238.4	242.8
Estimate of Fair Value, Fair Value Disclosure \| Level 2 \| Asset And Liability Management \| Foreign Exchange Contracts
Derivative Instruments
Assets	39.3	125.7
Liabilities	22.7	23.5
Estimate of Fair Value, Fair Value Disclosure \| Level 2 \| Asset And Liability Management \| Interest Rate Contracts
Derivative Instruments
Assets	143.7	126.8
Liabilities	29.4	30.5
Estimate of Fair Value, Fair Value Disclosure \| Level 2 \| Client Related and Trading \| Foreign Exchange Contracts
Derivative Instruments
Assets	2,779.0	4,149.5
Liabilities	2,751.9	4,072.0
Estimate of Fair Value, Fair Value Disclosure \| Level 2 \| Client Related and Trading \| Interest Rate Contracts
Derivative Instruments
Assets	139.1	105.5
Liabilities	136.1	101.3
Estimate of Fair Value, Fair Value Disclosure \| Level 3
Securities
Available for Sale	16.2	18.1
Loans (excluding Leases)
Held for Investment	32,245.0	30,600.4
Held for Sale	335.3	2.5
Other Liabilities
Standby Letters of Credit	38.1	60.1
Loan Commitments	24.6	$ 28.3
Estimate of Fair Value, Fair Value Disclosure \| Level 3 \| Asset And Liability Management \| Other Financial Derivatives (1)
Derivative Instruments
Liabilities	$ 10.9