Fair Value Measurements (Fair Value of Financial Instruments Not Measured at Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
ASSETS
Cash and Due from Banks	$ 4,677.2	$ 4,791.5
Federal Reserve and Other Central Bank Deposits	38,775.4	34,326.2
Interest-Bearing Deposits with Banks	1,944.7	1,939.0
Federal Funds Sold	426.0
Securities Purchased under Agreements to Resell	426.0	784.7
FAIR VALUE	20,654.5	24,473.0
LIABILITIES
Deposits	122,482.7	116,164.0
Federal Funds Purchased	2,159.5	3,045.4
Balance at December 31	462.0	784.7
Other Borrowings	6,521.0	6,567.8
Senior Notes	2,769.7	2,773.2
CARRYING  VALUE
ASSETS
Cash and Due from Banks	4,677.2	4,791.5
Federal Reserve and Other Central Bank Deposits	38,775.4	34,326.2
Interest-Bearing Deposits with Banks	1,944.7	1,939.0
Federal Funds Sold	25.0
Securities Purchased under Agreements to Resell	426.0	784.7
FAIR VALUE	22,296.7	26,221.7
Loans
Held for Investment	43,222.5	47,438.3
Other Assets	1,506.4	1,476.6
LIABILITIES
Deposits	122,482.7	116,164.0
Federal Funds Purchased	2,159.5	3,045.4
Balance at December 31	462.0	784.7
Other Borrowings	6,521.0	6,567.8
Senior Notes	2,769.7	2,773.2
Long-Term Debt	4,081.3	4,065.0
Unfunded Commitments	227.1	178.8
Other Liabilities	50.6	74.9
FAIR VALUE
ASSETS
Cash and Due from Banks	4,677.2	4,791.5
Federal Reserve and Other Central Bank Deposits	38,775.4	34,326.2
Interest-Bearing Deposits with Banks	1,944.7	1,939.0
Federal Funds Sold	25.0
Securities Purchased under Agreements to Resell	426.0	784.7
FAIR VALUE	20,654.5	24,473.0
Loans
Held for Investment	42,803.2	47,598.3
Other Assets	1,499.2	1,458.0
LIABILITIES
Deposits	122,536.5	116,207.6
Federal Funds Purchased	2,159.5	3,045.4
Balance at December 31	462.0	784.7
Other Borrowings	6,545.3	6,607.4
Senior Notes	2,800.3	2,798.1
Long-Term Debt	4,164.4	4,186.8
Unfunded Commitments	227.1	178.8
Other Liabilities	50.6	74.9
FAIR VALUE | LEVEL 1
ASSETS
Cash and Due from Banks	4,677.2	4,791.5
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
FAIR VALUE	0.0	0.0
Loans
Held for Investment	0.0	0.0
Other Assets	83.3	86.1
LIABILITIES
Deposits	0.0	0.0
Federal Funds Purchased	0.0	0.0
Balance at December 31	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
FAIR VALUE | LEVEL 2
ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	38,775.4	34,326.2
Interest-Bearing Deposits with Banks	1,944.7	1,939.0
Federal Funds Sold	25.0
Securities Purchased under Agreements to Resell	426.0	784.7
FAIR VALUE	20,654.5	24,473.0
Loans
Held for Investment	0.0	0.0
Other Assets	1,415.9	1,371.9
LIABILITIES
Deposits	122,536.5	116,207.6
Federal Funds Purchased	2,159.5	3,045.4
Balance at December 31	462.0	784.7
Other Borrowings	6,545.3	6,607.4
Senior Notes	2,800.3	2,798.1
Long-Term Debt	4,164.4	4,186.8
Unfunded Commitments	227.1	178.8
Other Liabilities	0.0	0.0
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	0.0
Securities Purchased under Agreements to Resell	0.0	0.0
FAIR VALUE	0.0	0.0
Loans
Held for Investment	42,803.2	47,598.3
Other Assets	0.0	0.0
LIABILITIES
Deposits	0.0	0.0
Federal Funds Purchased	0.0	0.0
Balance at December 31	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	$ 50.6	$ 74.9