Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 6,137.8
ONE TO FIVE YEARS	22,515.9
FIVE TO TEN YEARS	3,282.6
OVER TEN YEARS	1,038.0
AMORTIZED COST	32,974.3	$ 29,229.1
Available for Sale - Fair Value
ONE YEAR OR LESS	6,115.8
ONE TO FIVE YEARS	22,502.2
FIVE TO TEN YEARS	3,249.4
OVER TEN YEARS	1,021.8
Fair Value	32,889.2	29,001.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	6,656.7
ONE TO FIVE YEARS	11,647.7
FIVE TO TEN YEARS	3,000.4
OVER TEN YEARS	1,670.1
AMORTIZED COST	22,974.9	22,296.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	6,555.0
ONE TO FIVE YEARS	11,165.2
FIVE TO TEN YEARS	2,761.3
OVER TEN YEARS	1,356.8
Fair Value	21,838.3	20,654.5
Amortized Cost
ONE YEAR OR LESS	12,794.5
ONE TO FIVE YEARS	34,163.6
FIVE TO TEN YEARS	6,283.0
OVER TEN YEARS	2,708.1
AMORTIZED COST	55,949.2	51,525.8
Fair Value
ONE YEAR OR LESS	12,670.8
ONE TO FIVE YEARS	33,667.4
FIVE TO TEN YEARS	6,010.7
OVER TEN YEARS	2,378.6
FAIR VALUE	54,727.5	49,656.0
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	298.8
ONE TO FIVE YEARS	7,589.1
FIVE TO TEN YEARS	260.0
OVER TEN YEARS	0.0
AMORTIZED COST	8,147.9	7,388.9
Available for Sale - Fair Value
ONE YEAR OR LESS	298.9
ONE TO FIVE YEARS	7,596.2
FIVE TO TEN YEARS	259.9
OVER TEN YEARS	0.0
Fair Value	8,155.0	7,367.5
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	249.7
FIVE TO TEN YEARS	72.3
OVER TEN YEARS	0.0
AMORTIZED COST	322.0	311.2
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	241.2
FIVE TO TEN YEARS	69.5
OVER TEN YEARS	0.0
Fair Value	310.7	297.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	177.5
ONE TO FIVE YEARS	1,428.5
FIVE TO TEN YEARS	810.4
OVER TEN YEARS	59.6
AMORTIZED COST	2,476.0	2,548.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	177.3
ONE TO FIVE YEARS	1,427.2
FIVE TO TEN YEARS	799.1
OVER TEN YEARS	58.1
Fair Value	2,461.7	2,458.9
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	4,292.4
ONE TO FIVE YEARS	9,139.9
FIVE TO TEN YEARS	2,156.8
OVER TEN YEARS	916.2
AMORTIZED COST	16,505.3	13,410.5
Available for Sale - Fair Value
ONE YEAR OR LESS	4,282.4
ONE TO FIVE YEARS	9,120.7
FIVE TO TEN YEARS	2,138.6
OVER TEN YEARS	899.8
Fair Value	16,441.5	13,288.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	881.4
ONE TO FIVE YEARS	4,155.7
FIVE TO TEN YEARS	2,052.0
OVER TEN YEARS	1,423.3
AMORTIZED COST	8,512.4	8,635.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	807.8
ONE TO FIVE YEARS	3,841.5
FIVE TO TEN YEARS	1,835.0
OVER TEN YEARS	1,239.6
Fair Value	7,723.9	7,554.6
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	188.4
ONE TO FIVE YEARS	133.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	321.9	308.9
Available for Sale - Fair Value
ONE YEAR OR LESS	185.3
ONE TO FIVE YEARS	130.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	315.3	296.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,943.9
ONE TO FIVE YEARS	1,461.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	3,405.6	3,735.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,939.4
ONE TO FIVE YEARS	1,431.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	3,370.8	3,679.7
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	86.3
ONE TO FIVE YEARS	21.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	107.7	166.6
Available for Sale - Fair Value
ONE YEAR OR LESS	85.6
ONE TO FIVE YEARS	21.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	106.9	163.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	144.0
ONE TO FIVE YEARS	263.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	407.2	351.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	143.2
ONE TO FIVE YEARS	258.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	401.2	340.6
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	61.7
ONE TO FIVE YEARS	212.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	274.0	234.0
Available for Sale - Fair Value
ONE YEAR OR LESS	61.7
ONE TO FIVE YEARS	211.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	272.8	230.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	444.5
ONE TO FIVE YEARS	1,011.4
FIVE TO TEN YEARS	87.5
OVER TEN YEARS	0.0
AMORTIZED COST	1,543.4	1,776.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	443.0
ONE TO FIVE YEARS	967.2
FIVE TO TEN YEARS	87.2
OVER TEN YEARS	0.0
Fair Value	1,497.4	1,714.7
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	912.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	912.0	336.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	911.9
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	911.9	335.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	858.3
ONE TO FIVE YEARS	3,829.4
FIVE TO TEN YEARS	301.9
OVER TEN YEARS	0.0
AMORTIZED COST	4,989.6	4,617.0
Available for Sale - Fair Value
ONE YEAR OR LESS	854.0
ONE TO FIVE YEARS	3,839.2
FIVE TO TEN YEARS	293.5
OVER TEN YEARS	0.0
Fair Value	4,986.7	4,583.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	2,093.0
ONE TO FIVE YEARS	2,948.4
FIVE TO TEN YEARS	6.0
OVER TEN YEARS	0.0
AMORTIZED COST	5,047.4	4,146.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	2,073.5
ONE TO FIVE YEARS	2,889.9
FIVE TO TEN YEARS	5.1
OVER TEN YEARS	0.0
Fair Value	4,968.5	3,977.0
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	240.6
ONE TO FIVE YEARS	986.6
FIVE TO TEN YEARS	458.0
OVER TEN YEARS	121.8
AMORTIZED COST	1,807.0	2,188.6
Available for Sale - Fair Value
ONE YEAR OR LESS	236.9
ONE TO FIVE YEARS	991.8
FIVE TO TEN YEARS	459.5
OVER TEN YEARS	122.0
Fair Value	1,810.2	2,182.7
Held to Maturity - Amortized Cost
AMORTIZED COST		107.1
Held to Maturity - Fair Value
Fair Value		107.3
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	111.3
ONE TO FIVE YEARS	354.0
FIVE TO TEN YEARS	33.6
OVER TEN YEARS	0.0
AMORTIZED COST	498.9	603.4
Available for Sale - Fair Value
ONE YEAR OR LESS	111.0
ONE TO FIVE YEARS	350.7
FIVE TO TEN YEARS	28.4
OVER TEN YEARS	0.0
Fair Value	490.1	590.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	37.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	37.6	37.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	36.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	36.6	36.8
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	60.4
ONE TO FIVE YEARS	341.2
FIVE TO TEN YEARS	44.5
OVER TEN YEARS	187.2
AMORTIZED COST	633.3	621.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	58.9
ONE TO FIVE YEARS	313.4
FIVE TO TEN YEARS	34.9
OVER TEN YEARS	59.1
Fair Value	$ 466.3	$ 449.2