Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 4,510.8
ONE TO FIVE YEARS	20,524.2
FIVE TO TEN YEARS	4,620.8
OVER TEN YEARS	964.8
AMORTIZED COST	30,620.6	$ 29,229.1
Available for Sale - Fair Value
ONE YEAR OR LESS	4,484.2
ONE TO FIVE YEARS	20,452.1
FIVE TO TEN YEARS	4,581.5
OVER TEN YEARS	946.5
Fair Value	30,464.3	29,001.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	5,903.9
ONE TO FIVE YEARS	9,454.8
FIVE TO TEN YEARS	3,647.3
OVER TEN YEARS	1,868.3
AMORTIZED COST	20,874.3	22,296.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	5,776.1
ONE TO FIVE YEARS	8,869.0
FIVE TO TEN YEARS	3,326.0
OVER TEN YEARS	1,515.8
Fair Value	19,486.9	20,654.5
Amortized Cost
ONE YEAR OR LESS	10,414.7
ONE TO FIVE YEARS	29,979.0
FIVE TO TEN YEARS	8,268.1
OVER TEN YEARS	2,833.1
AMORTIZED COST	51,494.9	51,525.8
Fair Value
ONE YEAR OR LESS	10,260.3
ONE TO FIVE YEARS	29,321.1
FIVE TO TEN YEARS	7,907.5
OVER TEN YEARS	2,462.3
FAIR VALUE	49,951.2	49,656.0
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	396.3
ONE TO FIVE YEARS	6,165.9
FIVE TO TEN YEARS	1,447.6
OVER TEN YEARS	0.0
AMORTIZED COST	8,009.8	7,388.9
Available for Sale - Fair Value
ONE YEAR OR LESS	395.0
ONE TO FIVE YEARS	6,167.9
FIVE TO TEN YEARS	1,448.0
OVER TEN YEARS	0.0
Fair Value	8,010.9	7,367.5
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	246.7
FIVE TO TEN YEARS	70.6
OVER TEN YEARS	0.0
AMORTIZED COST	317.3	311.2
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	236.1
FIVE TO TEN YEARS	67.3
OVER TEN YEARS	0.0
Fair Value	303.4	297.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	130.9
ONE TO FIVE YEARS	1,430.2
FIVE TO TEN YEARS	854.6
OVER TEN YEARS	127.0
AMORTIZED COST	2,542.7	2,548.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	130.4
ONE TO FIVE YEARS	1,413.3
FIVE TO TEN YEARS	826.7
OVER TEN YEARS	121.2
Fair Value	2,491.6	2,458.9
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	3,296.8
ONE TO FIVE YEARS	7,886.8
FIVE TO TEN YEARS	2,327.4
OVER TEN YEARS	871.7
AMORTIZED COST	14,382.7	13,410.5
Available for Sale - Fair Value
ONE YEAR OR LESS	3,280.0
ONE TO FIVE YEARS	7,849.8
FIVE TO TEN YEARS	2,305.8
OVER TEN YEARS	853.3
Fair Value	14,288.9	13,288.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	905.5
ONE TO FIVE YEARS	3,392.8
FIVE TO TEN YEARS	2,633.6
OVER TEN YEARS	1,553.7
AMORTIZED COST	8,485.6	8,635.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	824.1
ONE TO FIVE YEARS	3,059.4
FIVE TO TEN YEARS	2,353.7
OVER TEN YEARS	1,338.8
Fair Value	7,576.0	7,554.6
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	55.0
ONE TO FIVE YEARS	255.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	310.1	308.9
Available for Sale - Fair Value
ONE YEAR OR LESS	55.1
ONE TO FIVE YEARS	245.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	300.4	296.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,836.2
ONE TO FIVE YEARS	856.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	2,692.3	3,735.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,831.2
ONE TO FIVE YEARS	812.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	2,643.3	3,679.7
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	83.1
ONE TO FIVE YEARS	40.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	123.6	166.6
Available for Sale - Fair Value
ONE YEAR OR LESS	82.0
ONE TO FIVE YEARS	39.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	121.6	163.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	207.0
ONE TO FIVE YEARS	141.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	348.2	351.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	205.6
ONE TO FIVE YEARS	133.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	339.3	340.6
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	234.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	234.8	234.0
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	232.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	232.5	230.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	182.3
ONE TO FIVE YEARS	1,166.9
FIVE TO TEN YEARS	83.9
OVER TEN YEARS	0.0
AMORTIZED COST	1,433.1	1,776.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	182.0
ONE TO FIVE YEARS	1,111.4
FIVE TO TEN YEARS	83.1
OVER TEN YEARS	0.0
Fair Value	1,376.5	1,714.7
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	463.6
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	463.6	336.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	463.4
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	463.4	335.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	464.9
ONE TO FIVE YEARS	4,095.6
FIVE TO TEN YEARS	297.0
OVER TEN YEARS	0.0
AMORTIZED COST	4,857.5	4,617.0
Available for Sale - Fair Value
ONE YEAR OR LESS	462.4
ONE TO FIVE YEARS	4,090.3
FIVE TO TEN YEARS	287.7
OVER TEN YEARS	0.0
Fair Value	4,840.4	4,583.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	2,121.5
ONE TO FIVE YEARS	2,012.5
FIVE TO TEN YEARS	5.5
OVER TEN YEARS	0.0
AMORTIZED COST	4,139.5	4,146.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	2,083.4
ONE TO FIVE YEARS	1,916.6
FIVE TO TEN YEARS	4.6
OVER TEN YEARS	0.0
Fair Value	4,004.6	3,977.0
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	184.3
ONE TO FIVE YEARS	1,114.4
FIVE TO TEN YEARS	444.5
OVER TEN YEARS	93.1
AMORTIZED COST	1,836.3	2,188.6
Available for Sale - Fair Value
ONE YEAR OR LESS	179.5
ONE TO FIVE YEARS	1,112.8
FIVE TO TEN YEARS	445.0
OVER TEN YEARS	93.2
Fair Value	1,830.5	2,182.7
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	82.7
FIVE TO TEN YEARS	23.3
OVER TEN YEARS	0.0
AMORTIZED COST	106.0	107.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	82.7
FIVE TO TEN YEARS	23.3
OVER TEN YEARS	0.0
Fair Value	106.0	107.3
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	30.4
ONE TO FIVE YEARS	484.4
FIVE TO TEN YEARS	33.7
OVER TEN YEARS	0.0
AMORTIZED COST	548.5	603.4
Available for Sale - Fair Value
ONE YEAR OR LESS	30.2
ONE TO FIVE YEARS	477.8
FIVE TO TEN YEARS	27.7
OVER TEN YEARS	0.0
Fair Value	535.7	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	36.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	36.4	36.8
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	56.9
ONE TO FIVE YEARS	334.8
FIVE TO TEN YEARS	46.4
OVER TEN YEARS	187.6
AMORTIZED COST	625.7	621.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	56.0
ONE TO FIVE YEARS	303.4
FIVE TO TEN YEARS	34.6
OVER TEN YEARS	55.8
Fair Value	$ 449.8	$ 449.2