Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 3,386.3
ONE TO FIVE YEARS	15,718.3
FIVE TO TEN YEARS	3,840.7
OVER TEN YEARS	713.7
AMORTIZED COST	23,659.0	$ 27,760.0
Available for Sale - Fair Value
ONE YEAR OR LESS	3,343.8
ONE YEAR OR LESS	15,307.0
FIVE TO TEN YEARS	3,752.4
OVER TEN YEARS	686.6
Total	23,089.8	26,699.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	6,756.6
ONE TO FIVE YEARS	12,455.8
FIVE TO TEN YEARS	4,798.3
FIVE TO TEN YEARS	2,211.0
Amortized Cost	26,221.7	25,036.1
Held to Maturity - Fair Value
Fair value, one year or less	6,618.0
Fair value, one to five years	11,607.2
Fair value, five to ten years	4,380.7
Fair value, over ten years	1,867.1
FAIR VALUE	24,473.0	22,879.3
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	198.3
ONE TO FIVE YEARS	3,483.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	3,681.5	2,837.7
Available for Sale - Fair Value
ONE YEAR OR LESS	197.8
ONE YEAR OR LESS	3,424.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Total	3,622.2	2,747.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
FIVE TO TEN YEARS	0.0
Amortized Cost	0.0	50.0
Held to Maturity - Fair Value
Fair value, one year or less	0.0
Fair value, one to five years	0.0
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	0.0	50.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	113.2
FIVE TO TEN YEARS	202.6
OVER TEN YEARS	0.0
AMORTIZED COST	315.8	817.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE YEAR OR LESS	106.8
FIVE TO TEN YEARS	189.0
OVER TEN YEARS	0.0
Total	295.8	787.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	45.7
ONE TO FIVE YEARS	1,100.7
FIVE TO TEN YEARS	1,126.3
FIVE TO TEN YEARS	291.2
Amortized Cost	2,563.9	2,565.3
Held to Maturity - Fair Value
Fair value, one year or less	45.3
Fair value, one to five years	1,076.8
Fair value, five to ten years	1,090.8
Fair value, over ten years	279.1
FAIR VALUE	2,492.0	2,415.5
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,384.3
ONE TO FIVE YEARS	5,330.2
FIVE TO TEN YEARS	3,327.0
OVER TEN YEARS	702.8
AMORTIZED COST	11,744.3	11,892.5
Available for Sale - Fair Value
ONE YEAR OR LESS	2,357.0
ONE YEAR OR LESS	5,237.5
FIVE TO TEN YEARS	3,282.8
OVER TEN YEARS	675.7
Total	11,553.0	11,545.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	984.3
ONE TO FIVE YEARS	3,381.4
FIVE TO TEN YEARS	3,248.6
FIVE TO TEN YEARS	1,741.0
Amortized Cost	9,355.3	9,407.7
Held to Maturity - Fair Value
Fair value, one year or less	885.4
Fair value, one to five years	3,037.6
Fair value, five to ten years	2,905.4
Fair value, over ten years	1,516.8
FAIR VALUE	8,345.2	8,331.7
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	67.9
ONE TO FIVE YEARS	216.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	284.8	387.6
Available for Sale - Fair Value
ONE YEAR OR LESS	66.1
ONE YEAR OR LESS	198.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Total	264.4	360.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	3,376.0
ONE TO FIVE YEARS	1,379.3
FIVE TO TEN YEARS	33.8
FIVE TO TEN YEARS	0.0
Amortized Cost	4,789.1
Held to Maturity - Fair Value
Fair value, one year or less	3,374.3
Fair value, one to five years	1,294.7
Fair value, five to ten years	29.6
Fair value, over ten years	0.0
FAIR VALUE	4,698.6
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	105.2
ONE TO FIVE YEARS	182.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	287.5	1,774.3
Available for Sale - Fair Value
ONE YEAR OR LESS	103.8
ONE YEAR OR LESS	175.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Total	279.5	1,747.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	276.8
ONE TO FIVE YEARS	353.7
FIVE TO TEN YEARS	15.6
FIVE TO TEN YEARS	0.0
Amortized Cost	646.1	713.3
Held to Maturity - Fair Value
Fair value, one year or less	269.5
Fair value, one to five years	335.2
Fair value, five to ten years	13.2
Fair value, over ten years	0.0
FAIR VALUE	617.9	667.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	90.3
ONE TO FIVE YEARS	266.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	356.8	403.1
Available for Sale - Fair Value
ONE YEAR OR LESS	89.2
ONE YEAR OR LESS	257.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Total	347.1	388.7
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	345.4
ONE TO FIVE YEARS	1,623.1
FIVE TO TEN YEARS	240.1
FIVE TO TEN YEARS	0.0
Amortized Cost	2,208.6	2,530.3
Held to Maturity - Fair Value
Fair value, one year or less	342.1
Fair value, one to five years	1,542.5
Fair value, five to ten years	216.0
Fair value, over ten years	0.0
FAIR VALUE	2,100.6	2,371.9
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	585.1
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
FIVE TO TEN YEARS	0.0
Amortized Cost	585.1	35.9
Held to Maturity - Fair Value
Fair value, one year or less	584.4
Fair value, one to five years	0.0
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	584.4	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	280.3
ONE TO FIVE YEARS	2,665.8
FIVE TO TEN YEARS	67.7
OVER TEN YEARS	0.0
AMORTIZED COST	3,013.8	2,645.8
Available for Sale - Fair Value
ONE YEAR OR LESS	277.8
ONE YEAR OR LESS	2,567.1
FIVE TO TEN YEARS	55.0
OVER TEN YEARS	0.0
Total	2,899.9	2,479.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,080.8
ONE TO FIVE YEARS	4,159.0
FIVE TO TEN YEARS	5.7
FIVE TO TEN YEARS	0.0
Amortized Cost	5,245.5	5,703.3
Held to Maturity - Fair Value
Fair value, one year or less	1,055.6
Fair value, one to five years	3,893.6
Fair value, five to ten years	4.6
Fair value, over ten years	0.0
FAIR VALUE	4,953.8	5,268.2
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	218.1
ONE TO FIVE YEARS	2,724.2
FIVE TO TEN YEARS	107.8
OVER TEN YEARS	10.9
AMORTIZED COST	3,061.0	5,544.3
Available for Sale - Fair Value
ONE YEAR OR LESS	212.8
ONE YEAR OR LESS	2,631.1
FIVE TO TEN YEARS	107.8
OVER TEN YEARS	10.9
Total	2,962.6	5,256.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	14.7
ONE TO FIVE YEARS	102.0
FIVE TO TEN YEARS	97.5
FIVE TO TEN YEARS	0.0
Amortized Cost	214.2	263.7
Held to Maturity - Fair Value
Fair value, one year or less	14.7
Fair value, one to five years	102.1
Fair value, five to ten years	97.6
Fair value, over ten years	0.0
FAIR VALUE	214.4	262.7
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	41.9
ONE TO FIVE YEARS	736.0
FIVE TO TEN YEARS	135.6
OVER TEN YEARS	0.0
AMORTIZED COST	913.5	1,456.9
Available for Sale - Fair Value
ONE YEAR OR LESS	39.3
ONE YEAR OR LESS	708.2
FIVE TO TEN YEARS	117.8
OVER TEN YEARS	0.0
Total	865.3	1,387.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	37.6
FIVE TO TEN YEARS	0.0
FIVE TO TEN YEARS	0.0
Amortized Cost	37.6
Held to Maturity - Fair Value
Fair value, one year or less	0.0
Fair value, one to five years	36.8
Fair value, five to ten years	0.0
Fair value, over ten years	0.0
FAIR VALUE	36.8
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	47.8
ONE TO FIVE YEARS	319.0
FIVE TO TEN YEARS	30.7
FIVE TO TEN YEARS	178.8
Amortized Cost	576.3	532.6
Held to Maturity - Fair Value
Fair value, one year or less	46.7
Fair value, one to five years	287.9
Fair value, five to ten years	23.5
Fair value, over ten years	71.2
FAIR VALUE	$ 429.3	$ 375.2