Securities (Reconciliation of Amortized Cost to Fair Value of Available For Sale and Held to Maturity Debt Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Available for Sale Debt Securities
AMORTIZED COST	$ 30,620.6	$ 29,229.1
GROSS UNREALIZED GAINS	50.6	28.8
GROSS UNREALIZED LOSSES	206.9	256.4
FAIR VALUE	30,464.3	29,001.5
Held to Maturity Debt Securities
AMORTIZED COST	20,874.3	22,296.7
GROSS UNREALIZED GAINS	3.7	2.9
GROSS UNREALIZED LOSSES	1,391.1	1,645.1
FAIR VALUE	19,486.9	20,654.5
AMORTIZED COST	51,494.9	51,525.8
GROSS UNREALIZED GAINS	54.3	31.7
GROSS UNREALIZED LOSSES	1,598.0	1,901.5
FAIR VALUE	49,951.2	49,656.0
U.S. Government
Available for Sale Debt Securities
AMORTIZED COST	8,009.8	7,388.9
GROSS UNREALIZED GAINS	12.0	1.5
GROSS UNREALIZED LOSSES	10.9	22.9
FAIR VALUE	8,010.9	7,367.5
Obligations of States and Political Subdivisions
Available for Sale Debt Securities
AMORTIZED COST	317.3	311.2
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	13.9	13.6
FAIR VALUE	303.4	297.6
Held to Maturity Debt Securities
AMORTIZED COST	2,542.7	2,548.2
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	51.1	89.3
FAIR VALUE	2,491.6	2,458.9
Government Sponsored Agency
Available for Sale Debt Securities
AMORTIZED COST	14,382.7	13,410.5
GROSS UNREALIZED GAINS	16.2	10.9
GROSS UNREALIZED LOSSES	110.0	132.5
FAIR VALUE	14,288.9	13,288.9
Held to Maturity Debt Securities
AMORTIZED COST	8,485.6	8,635.0
GROSS UNREALIZED GAINS	1.0	0.9
GROSS UNREALIZED LOSSES	910.6	1,081.3
FAIR VALUE	7,576.0	7,554.6
Non-U.S. Government
Available for Sale Debt Securities
AMORTIZED COST	310.1	308.9
GROSS UNREALIZED GAINS	0.5	0.2
GROSS UNREALIZED LOSSES	10.2	12.3
FAIR VALUE	300.4	296.8
Held to Maturity Debt Securities
AMORTIZED COST	2,692.3	3,735.8
GROSS UNREALIZED GAINS	0.2	0.2
GROSS UNREALIZED LOSSES	49.2	56.3
FAIR VALUE	2,643.3	3,679.7
Corporate Debt
Available for Sale Debt Securities
AMORTIZED COST	123.6	166.6
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	2.0	2.9
FAIR VALUE	121.6	163.8
Held to Maturity Debt Securities
AMORTIZED COST	348.2	351.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	8.9	11.0
FAIR VALUE	339.3	340.6
Covered Bonds
Available for Sale Debt Securities
AMORTIZED COST	234.8	234.0
GROSS UNREALIZED GAINS	0.5	0.5
GROSS UNREALIZED LOSSES	2.8	3.6
FAIR VALUE	232.5	230.9
Held to Maturity Debt Securities
AMORTIZED COST	1,433.1	1,776.8
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	56.7	62.2
FAIR VALUE	1,376.5	1,714.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale Debt Securities
AMORTIZED COST	4,857.5	4,617.0
GROSS UNREALIZED GAINS	17.6	8.3
GROSS UNREALIZED LOSSES	34.7	42.2
FAIR VALUE	4,840.4	4,583.1
Held to Maturity Debt Securities
AMORTIZED COST	4,139.5	4,146.9
GROSS UNREALIZED GAINS	2.4	1.4
GROSS UNREALIZED LOSSES	137.3	171.3
FAIR VALUE	4,004.6	3,977.0
Other Asset-Backed
Available for Sale Debt Securities
AMORTIZED COST	1,836.3	2,188.6
GROSS UNREALIZED GAINS	3.8	7.0
GROSS UNREALIZED LOSSES	9.6	12.9
FAIR VALUE	1,830.5	2,182.7
Held to Maturity Debt Securities
AMORTIZED COST	106.0	107.1
GROSS UNREALIZED GAINS	0.0	0.3
GROSS UNREALIZED LOSSES	0.0	0.1
FAIR VALUE	106.0	107.3
Commercial Mortgage-Backed
Available for Sale Debt Securities
AMORTIZED COST	548.5	603.4
GROSS UNREALIZED GAINS	0.0	0.3
GROSS UNREALIZED LOSSES	12.8	13.5
FAIR VALUE	535.7	590.2
Held to Maturity Debt Securities
AMORTIZED COST	37.6	37.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.2	0.8
FAIR VALUE	36.4	36.8
Certificates of Deposit
Held to Maturity Debt Securities
AMORTIZED COST	463.6	336.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.2	0.3
FAIR VALUE	463.4	335.7
Other
Held to Maturity Debt Securities
AMORTIZED COST	625.7	621.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	175.9	172.5
FAIR VALUE	$ 449.8	$ 449.2