Fair Value Measurements (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
FINANCIAL ASSETS
Cash and Due from Banks	$ 5,816.0	$ 5,873.1
Federal Reserve and Other Central Bank Deposits	41,836.1	53,524.9
Interest-Bearing Deposits with Banks	1,856.0	1,729.4
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,678.9	2,654.1
Debt Securities - Held to Maturity	23,138.0	22,381.2
FINANCIAL LIABILITIES
Deposits	139,698.2	142,797.7
Federal Funds Purchased	1,974.3	2,141.1
Securities Sold Under Agreements to Repurchase	330.5	292.2
Other Borrowings	7,839.5	7,158.3
Senior Notes	3,345.5	3,351.5
Unfunded Commitments	72.6	19.6
BOOK VALUE
FINANCIAL ASSETS
Cash and Due from Banks	5,816.0	5,873.1
Federal Reserve and Other Central Bank Deposits	41,836.1	53,524.9
Interest-Bearing Deposits with Banks	1,856.0	1,729.4
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,678.9	2,654.1
Debt Securities - Held to Maturity	24,286.2	23,429.6
Loans
Held for Investment	42,334.4	41,777.1
Held for Sale	10.0	6.8
Other Assets	1,654.2	1,668.6
FINANCIAL LIABILITIES
Deposits	139,698.2	142,797.7
Federal Funds Purchased	1,974.3	2,141.1
Securities Sold Under Agreements to Repurchase	330.5	292.2
Other Borrowings	7,839.5	7,158.3
Senior Notes	3,345.5	3,351.5
Long-Term Debt	2,881.6	3,484.4
Unfunded Commitments	364.9	373.0
Other Liabilities	39.7	37.9
TOTAL ESTIMATED FAIR VALUE
FINANCIAL ASSETS
Cash and Due from Banks	5,816.0	5,873.1
Federal Reserve and Other Central Bank Deposits	41,836.1	53,524.9
Interest-Bearing Deposits with Banks	1,856.0	1,729.4
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,678.9	2,654.1
Debt Securities - Held to Maturity	23,138.0	22,381.2
Loans
Held for Investment	51,320.7	41,661.2
Held for Sale	10.0	6.8
Other Assets	1,650.2	1,664.8
FINANCIAL LIABILITIES
Deposits	139,733.9	142,348.6
Federal Funds Purchased	1,974.3	2,141.1
Securities Sold Under Agreements to Repurchase	330.5	292.2
Other Borrowings	7,858.2	7,185.5
Senior Notes	3,386.9	3,405.5
Long-Term Debt	2,947.3	3,596.8
Unfunded Commitments	364.9	373.0
Other Liabilities	39.7	37.9
TOTAL ESTIMATED FAIR VALUE | LEVEL 1
FINANCIAL ASSETS
Cash and Due from Banks	5,816.0	5,873.1
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 - Held to Maturity	0.0	0.0
Loans
Held for Investment	0.0	0.0
Held for Sale	0.0	0.0
Other Assets	91.5	86.3
FINANCIAL LIABILITIES
Deposits	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
FINANCIAL ASSETS
Cash and Due from Banks	0.0	0.0
Federal Reserve and Other Central Bank Deposits	41,836.1	53,524.9
Interest-Bearing Deposits with Banks	1,856.0	1,729.4
Federal Funds Sold and Securities Purchased under Agreements to Resell	1,678.9	2,654.1
Debt Securities - Held to Maturity	23,138.0	22,381.2
Loans
Held for Investment	0.0	0.0
Held for Sale	10.0	6.8
Other Assets	1,558.7	1,578.5
FINANCIAL LIABILITIES
Deposits	139,733.9	142,348.6
Federal Funds Purchased	1,974.3	2,141.1
Securities Sold Under Agreements to Repurchase	330.5	292.2
Other Borrowings	7,858.2	7,185.5
Senior Notes	3,386.9	3,405.5
Long-Term Debt	2,947.3	3,596.8
Unfunded Commitments	364.9	373.0
Other Liabilities	0.0	0.0
TOTAL ESTIMATED FAIR VALUE | LEVEL 3
FINANCIAL 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 - Held to Maturity	0.0	0.0
Loans
Held for Investment	51,320.7	41,661.2
Held for Sale	0.0	0.0
Other Assets	0.0	0.0
FINANCIAL LIABILITIES
Deposits	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	$ 39.7	$ 37.9