Securities (Reconciliation of Amortized Cost to Fair Value of Available For Sale and Held to Maturity Debt Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Available for Sale Debt Securities
AMORTIZED COST	$ 32,974.3	$ 29,229.1
GROSS UNREALIZED GAINS	83.7	28.8
GROSS UNREALIZED LOSSES	168.8	256.4
FAIR VALUE	32,889.2	29,001.5
Held to Maturity Debt Securities
AMORTIZED COST	22,974.9	22,296.7
GROSS UNREALIZED GAINS	13.9	2.9
GROSS UNREALIZED LOSSES	1,150.5	1,645.1
FAIR VALUE	21,838.3	20,654.5
AMORTIZED COST	55,949.2	51,525.8
GROSS UNREALIZED GAINS	97.6	31.7
GROSS UNREALIZED LOSSES	1,319.3	1,901.5
FAIR VALUE	54,727.5	49,656.0
U.S. Government
Available for Sale Debt Securities
AMORTIZED COST	8,147.9	7,388.9
GROSS UNREALIZED GAINS	15.6	1.5
GROSS UNREALIZED LOSSES	8.5	22.9
FAIR VALUE	8,155.0	7,367.5
Obligations of States and Political Subdivisions
Available for Sale Debt Securities
AMORTIZED COST	322.0	311.2
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	11.3	13.6
FAIR VALUE	310.7	297.6
Held to Maturity Debt Securities
AMORTIZED COST	2,476.0	2,548.2
GROSS UNREALIZED GAINS	2.6	0.0
GROSS UNREALIZED LOSSES	16.9	89.3
FAIR VALUE	2,461.7	2,458.9
Government Sponsored Agency
Available for Sale Debt Securities
AMORTIZED COST	16,505.3	13,410.5
GROSS UNREALIZED GAINS	35.6	10.9
GROSS UNREALIZED LOSSES	99.4	132.5
FAIR VALUE	16,441.5	13,288.9
Held to Maturity Debt Securities
AMORTIZED COST	8,512.4	8,635.0
GROSS UNREALIZED GAINS	7.4	0.9
GROSS UNREALIZED LOSSES	795.9	1,081.3
FAIR VALUE	7,723.9	7,554.6
Non-U.S. Government
Available for Sale Debt Securities
AMORTIZED COST	321.9	308.9
GROSS UNREALIZED GAINS	0.7	0.2
GROSS UNREALIZED LOSSES	7.3	12.3
FAIR VALUE	315.3	296.8
Held to Maturity Debt Securities
AMORTIZED COST	3,405.6	3,735.8
GROSS UNREALIZED GAINS	0.3	0.2
GROSS UNREALIZED LOSSES	35.1	56.3
FAIR VALUE	3,370.8	3,679.7
Corporate Debt
Available for Sale Debt Securities
AMORTIZED COST	107.7	166.6
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	0.8	2.9
FAIR VALUE	106.9	163.8
Held to Maturity Debt Securities
AMORTIZED COST	407.2	351.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	6.0	11.0
FAIR VALUE	401.2	340.6
Covered Bonds
Available for Sale Debt Securities
AMORTIZED COST	274.0	234.0
GROSS UNREALIZED GAINS	0.6	0.5
GROSS UNREALIZED LOSSES	1.8	3.6
FAIR VALUE	272.8	230.9
Held to Maturity Debt Securities
AMORTIZED COST	1,543.4	1,776.8
GROSS UNREALIZED GAINS	0.2	0.1
GROSS UNREALIZED LOSSES	46.2	62.2
FAIR VALUE	1,497.4	1,714.7
Certificates of Deposit
Held to Maturity Debt Securities
AMORTIZED COST	912.0	336.0
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	0.2	0.3
FAIR VALUE	911.9	335.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale Debt Securities
AMORTIZED COST	4,989.6	4,617.0
GROSS UNREALIZED GAINS	23.5	8.3
GROSS UNREALIZED LOSSES	26.4	42.2
FAIR VALUE	4,986.7	4,583.1
Held to Maturity Debt Securities
AMORTIZED COST	5,047.4	4,146.9
GROSS UNREALIZED GAINS	3.3	1.4
GROSS UNREALIZED LOSSES	82.2	171.3
FAIR VALUE	4,968.5	3,977.0
Other Asset-Backed
Available for Sale Debt Securities
AMORTIZED COST	1,807.0	2,188.6
GROSS UNREALIZED GAINS	7.7	7.0
GROSS UNREALIZED LOSSES	4.5	12.9
FAIR VALUE	1,810.2	2,182.7
Held to Maturity Debt Securities
AMORTIZED COST		107.1
GROSS UNREALIZED GAINS		0.3
GROSS UNREALIZED LOSSES		0.1
FAIR VALUE		107.3
Commercial Mortgage-Backed
Available for Sale Debt Securities
AMORTIZED COST	498.9	603.4
GROSS UNREALIZED GAINS	0.0	0.3
GROSS UNREALIZED LOSSES	8.8	13.5
FAIR VALUE	490.1	590.2
Held to Maturity Debt Securities
AMORTIZED COST	37.6	37.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.0	0.8
FAIR VALUE	36.6	36.8
Other
Held to Maturity Debt Securities
AMORTIZED COST	633.3	621.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	167.0	172.5
FAIR VALUE	$ 466.3	$ 449.2