Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 4,274.1
ONE TO FIVE YEARS	17,685.9
FIVE TO TEN YEARS	5,564.3
OVER TEN YEARS	988.8
AMORTIZED COST	28,513.1	$ 23,659.0
Available for Sale - Fair Value
ONE YEAR OR LESS	4,252.4
ONE TO FIVE YEARS	17,573.1
FIVE TO TEN YEARS	5,515.8
OVER TEN YEARS	970.6
FAIR VALUE	28,311.9	23,089.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	5,531.2
ONE TO FIVE YEARS	10,933.3
FIVE TO TEN YEARS	4,163.8
OVER TEN YEARS	2,080.3
Amortized Cost	22,708.6	26,221.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	5,426.7
ONE TO FIVE YEARS	10,316.7
FIVE TO TEN YEARS	3,873.8
OVER TEN YEARS	1,754.8
Fair Value	21,372.0	24,473.0
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	496.2
ONE TO FIVE YEARS	3,862.0
FIVE TO TEN YEARS	2,139.2
OVER TEN YEARS	0.0
AMORTIZED COST	6,497.4	3,681.5
Available for Sale - Fair Value
ONE YEAR OR LESS	493.9
ONE TO FIVE YEARS	3,852.1
FIVE TO TEN YEARS	2,135.7
OVER TEN YEARS	0.0
FAIR VALUE	6,481.7	3,622.2
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	206.3
FIVE TO TEN YEARS	115.3
OVER TEN YEARS	0.0
AMORTIZED COST	321.6	315.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	198.0
FIVE TO TEN YEARS	108.0
OVER TEN YEARS	0.0
FAIR VALUE	306.0	295.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	106.8
ONE TO FIVE YEARS	1,302.2
FIVE TO TEN YEARS	935.6
OVER TEN YEARS	205.3
Amortized Cost	2,549.9	2,563.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	106.3
ONE TO FIVE YEARS	1,298.7
FIVE TO TEN YEARS	931.9
OVER TEN YEARS	202.6
Fair Value	2,539.5	2,492.0
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	3,223.6
ONE TO FIVE YEARS	7,181.0
FIVE TO TEN YEARS	2,472.3
OVER TEN YEARS	872.1
AMORTIZED COST	13,749.0	11,744.3
Available for Sale - Fair Value
ONE YEAR OR LESS	3,209.3
ONE TO FIVE YEARS	7,141.1
FIVE TO TEN YEARS	2,449.7
OVER TEN YEARS	853.6
FAIR VALUE	13,653.7	11,553.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	917.4
ONE TO FIVE YEARS	3,345.6
FIVE TO TEN YEARS	2,874.6
OVER TEN YEARS	1,689.2
Amortized Cost	8,826.8	9,355.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	843.7
ONE TO FIVE YEARS	3,056.6
FIVE TO TEN YEARS	2,617.6
OVER TEN YEARS	1,487.7
Fair Value	8,005.6	8,345.2
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	69.2
ONE TO FIVE YEARS	272.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	341.5	284.8
Available for Sale - Fair Value
ONE YEAR OR LESS	68.8
ONE TO FIVE YEARS	258.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	327.1	264.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,254.9
ONE TO FIVE YEARS	1,072.4
FIVE TO TEN YEARS	34.0
OVER TEN YEARS	0.0
Amortized Cost	2,361.3	4,789.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,252.6
ONE TO FIVE YEARS	1,013.1
FIVE TO TEN YEARS	30.6
OVER TEN YEARS	0.0
Fair Value	2,296.3	4,698.6
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	114.6
ONE TO FIVE YEARS	90.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	205.0	287.5
Available for Sale - Fair Value
ONE YEAR OR LESS	114.1
ONE TO FIVE YEARS	87.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	201.1	279.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	249.3
ONE TO FIVE YEARS	226.1
FIVE TO TEN YEARS	15.7
OVER TEN YEARS	0.0
Amortized Cost	491.1	646.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	245.5
ONE TO FIVE YEARS	217.2
FIVE TO TEN YEARS	13.7
OVER TEN YEARS	0.0
Fair Value	476.4	617.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	242.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	242.2	356.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	239.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	239.7	347.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	660.6
ONE TO FIVE YEARS	1,254.4
FIVE TO TEN YEARS	209.8
OVER TEN YEARS	0.0
Amortized Cost	2,124.8	2,208.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	656.9
ONE TO FIVE YEARS	1,200.5
FIVE TO TEN YEARS	195.0
OVER TEN YEARS	0.0
Fair Value	2,052.4	2,100.6
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	542.6
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	542.6	585.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	542.3
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	542.3	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	192.8
ONE TO FIVE YEARS	3,520.1
FIVE TO TEN YEARS	306.8
OVER TEN YEARS	0.0
AMORTIZED COST	4,019.7	3,013.8
Available for Sale - Fair Value
ONE YEAR OR LESS	190.9
ONE TO FIVE YEARS	3,501.1
FIVE TO TEN YEARS	296.1
OVER TEN YEARS	0.0
FAIR VALUE	3,988.1	2,899.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,723.9
ONE TO FIVE YEARS	3,332.5
FIVE TO TEN YEARS	5.7
OVER TEN YEARS	0.0
Amortized Cost	5,062.1	5,245.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,705.6
ONE TO FIVE YEARS	3,156.7
FIVE TO TEN YEARS	4.8
OVER TEN YEARS	0.0
Fair Value	4,867.1	4,953.8
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	154.6
ONE TO FIVE YEARS	1,726.6
FIVE TO TEN YEARS	496.6
OVER TEN YEARS	116.7
AMORTIZED COST	2,494.5	3,061.0
Available for Sale - Fair Value
ONE YEAR OR LESS	152.6
ONE TO FIVE YEARS	1,719.6
FIVE TO TEN YEARS	498.2
OVER TEN YEARS	117.0
FAIR VALUE	2,487.4	2,962.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	4.7
ONE TO FIVE YEARS	69.2
FIVE TO TEN YEARS	37.7
OVER TEN YEARS	0.0
Amortized Cost	111.6	214.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	4.5
ONE TO FIVE YEARS	69.7
FIVE TO TEN YEARS	37.7
OVER TEN YEARS	0.0
Fair Value	111.9	214.4
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	23.1
ONE TO FIVE YEARS	585.0
FIVE TO TEN YEARS	34.1
OVER TEN YEARS	0.0
AMORTIZED COST	642.2	913.5
Available for Sale - Fair Value
ONE YEAR OR LESS	22.8
ONE TO FIVE YEARS	576.2
FIVE TO TEN YEARS	28.1
OVER TEN YEARS	0.0
FAIR VALUE	627.1	865.3
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	71.0
ONE TO FIVE YEARS	293.3
FIVE TO TEN YEARS	50.7
OVER TEN YEARS	185.8
Amortized Cost	600.8	576.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	69.3
ONE TO FIVE YEARS	267.6
FIVE TO TEN YEARS	42.5
OVER TEN YEARS	64.5
Fair Value	$ 443.9	$ 429.3