Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
ASSETS
Cash and Due from Banks	$ 3,783.2	$ 3,056.8
Federal Reserve and Other Central Bank Deposits	55,695.7	64,582.2
Interest-Bearing Deposits with Banks	2,811.1	1,949.4
Securities Purchased under Agreements to Resell	1,031.4	686.4
Debt Securities - Held to Maturity	20,399.1	23,315.4
Deposits
Savings Certificates and Other Time	846.0	842.7
Non U.S. Offices Interest-Bearing	69,541.5	75,767.1
Federal Funds Purchased	0.2	0.2
Securities Sold Under Agreements to Repurchase	329.7	531.9
Other Borrowings	3,521.1	3,583.8
Senior Notes	2,377.5	2,505.5
BOOK VALUE
ASSETS
Cash and Due from Banks	3,783.2	3,056.8
Federal Reserve and Other Central Bank Deposits	55,695.7	64,582.2
Interest-Bearing Deposits with Banks	2,811.1	1,949.4
Securities Purchased under Agreements to Resell	1,031.4	686.4
Debt Securities - Held to Maturity	21,223.5	23,564.5
Loans (excluding Leases)
Held for Investment	39,081.0	40,319.3
Held for Sale	10.8	12.3
Other Assets	1,292.9	1,275.3
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	79,242.1	83,318.6
Savings Certificates and Other Time	846.0	842.7
Non U.S. Offices Interest-Bearing	69,541.5	75,767.1
Federal Funds Purchased	0.2	0.2
Securities Sold Under Agreements to Repurchase	329.7	531.9
Other Borrowings	3,521.1	3,583.8
Senior Notes	2,377.5	2,505.5
Long-Term Debt	1,106.9	1,145.7
Unfunded Commitments	297.4	289.3
Other Liabilities	79.5	75.5
TOTAL ESTIMATED FAIR VALUE
ASSETS
Cash and Due from Banks	3,783.2	3,056.8
Federal Reserve and Other Central Bank Deposits	55,695.7	64,582.2
Interest-Bearing Deposits with Banks	2,811.1	1,949.4
Securities Purchased under Agreements to Resell	1,031.4	686.4
Debt Securities - Held to Maturity	20,399.1	23,315.4
Loans (excluding Leases)
Held for Investment	38,687.2	40,208.2
Held for Sale	22.3	24.5
Other Assets	1,292.1	1,279.8
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	79,242.1	83,318.6
Savings Certificates and Other Time	841.2	843.8
Non U.S. Offices Interest-Bearing	69,541.5	75,767.1
Federal Funds Purchased	0.2	0.2
Securities Sold Under Agreements to Repurchase	329.7	531.9
Other Borrowings	3,520.1	3,583.9
Senior Notes	2,432.2	2,591.4
Long-Term Debt	1,114.6	1,189.4
Unfunded Commitments	297.4	289.3
Other Liabilities	79.5	75.5
TOTAL ESTIMATED FAIR VALUE | LEVEL 1
ASSETS
Cash and Due from Banks	3,783.2	3,056.8
Federal Reserve and Other Central Bank Deposits	0.0	0.0
Interest-Bearing Deposits with Banks	0.0	0.0
Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities - Held to Maturity	49.0	47.0
Loans (excluding Leases)
Held for Investment	0.0	0.0
Held for Sale	0.0	0.0
Other Assets	130.9	119.1
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	79,242.1	83,318.6
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	0.0	0.0
Unfunded Commitments	0.0	0.0
Other Liabilities	0.0	0.0
TOTAL ESTIMATED FAIR VALUE | LEVEL 2
ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	55,695.7	64,582.2
Interest-Bearing Deposits with Banks	2,811.1	1,949.4
Securities Purchased under Agreements to Resell	1,031.4	686.4
Debt Securities - Held to Maturity	20,350.1	23,268.4
Loans (excluding Leases)
Held for Investment	0.0	0.0
Held for Sale	22.3	24.5
Other Assets	1,161.2	1,160.7
Deposits
Demand, Noninterest-Bearing, Savings, Money Market and Other Interest-Bearing	0.0	0.0
Savings Certificates and Other Time	841.2	843.8
Non U.S. Offices Interest-Bearing	69,541.5	75,767.1
Federal Funds Purchased	0.2	0.2
Securities Sold Under Agreements to Repurchase	329.7	531.9
Other Borrowings	3,520.1	3,583.9
Senior Notes	2,432.2	2,591.4
Long-Term Debt	1,114.6	1,189.4
Unfunded Commitments	297.4	289.3
Other Liabilities	0.0	0.0
TOTAL ESTIMATED 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
Securities Purchased under Agreements to Resell	0.0	0.0
Debt Securities - Held to Maturity	0.0	0.0
Loans (excluding Leases)
Held for Investment	38,687.2	40,208.2
Held for Sale	0.0	0.0
Other Assets	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	0.0	0.0
Unfunded Commitments	0.0	0.0
Other Liabilities	$ 79.5	$ 75.5