Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 4,224.2
ONE TO FIVE YEARS	15,921.3
FIVE TO TEN YEARS	4,147.4
OVER TEN YEARS	953.2
AMORTIZED COST	25,246.1	$ 27,760.0
Available for Sale - Fair Value
ONE YEAR OR LESS	4,146.4
ONE TO FIVE YEARS	15,297.3
FIVE TO TEN YEARS	3,995.7
OVER TEN YEARS	902.7
FAIR VALUE	24,342.1	26,699.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	4,791.9
ONE TO FIVE YEARS	12,712.2
FIVE TO TEN YEARS	5,107.9
OVER TEN YEARS	2,308.4
Amortized Cost	24,920.4	25,036.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	4,642.2
ONE TO FIVE YEARS	11,529.1
FIVE TO TEN YEARS	4,444.4
OVER TEN YEARS	1,834.0
Fair Value	22,449.7	22,879.3
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	297.3
ONE TO FIVE YEARS	2,683.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	2,981.2	2,837.7
Available for Sale - Fair Value
ONE YEAR OR LESS	296.4
ONE TO FIVE YEARS	2,583.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	2,879.4	2,747.4
Held to Maturity - Amortized Cost
Amortized Cost		50.0
Held to Maturity - Fair Value
Fair Value		50.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	109.1
FIVE TO TEN YEARS	192.4
OVER TEN YEARS	0.0
AMORTIZED COST	301.5	817.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	102.8
FIVE TO TEN YEARS	179.3
OVER TEN YEARS	0.0
FAIR VALUE	282.1	787.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	58.1
ONE TO FIVE YEARS	988.1
FIVE TO TEN YEARS	1,187.1
OVER TEN YEARS	344.4
Amortized Cost	2,577.7	2,565.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	56.9
ONE TO FIVE YEARS	941.6
FIVE TO TEN YEARS	1,095.1
OVER TEN YEARS	306.2
Fair Value	2,399.8	2,415.5
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,294.8
ONE TO FIVE YEARS	5,259.3
FIVE TO TEN YEARS	3,481.1
OVER TEN YEARS	952.9
AMORTIZED COST	11,988.1	11,892.5
Available for Sale - Fair Value
ONE YEAR OR LESS	2,250.7
ONE TO FIVE YEARS	5,113.8
FIVE TO TEN YEARS	3,389.9
OVER TEN YEARS	902.4
FAIR VALUE	11,656.8	11,545.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	859.7
ONE TO FIVE YEARS	3,105.8
FIVE TO TEN YEARS	3,408.4
OVER TEN YEARS	1,786.0
Amortized Cost	9,159.9	9,407.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	727.6
ONE TO FIVE YEARS	2,649.4
FIVE TO TEN YEARS	2,901.4
OVER TEN YEARS	1,475.9
Fair Value	7,754.3	8,331.7
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	271.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	271.0	387.6
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	246.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	246.9	360.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	2,341.3
ONE TO FIVE YEARS	1,338.1
FIVE TO TEN YEARS	32.4
OVER TEN YEARS	0.0
Amortized Cost	3,711.8	3,234.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	2,337.5
ONE TO FIVE YEARS	1,221.1
FIVE TO TEN YEARS	26.7
OVER TEN YEARS	0.0
Fair Value	3,585.3	3,100.3
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	150.4
ONE TO FIVE YEARS	230.9
FIVE TO TEN YEARS	16.6
OVER TEN YEARS	0.0
AMORTIZED COST	397.9	1,774.3
Available for Sale - Fair Value
ONE YEAR OR LESS	148.2
ONE TO FIVE YEARS	218.8
FIVE TO TEN YEARS	13.9
OVER TEN YEARS	0.0
FAIR VALUE	380.9	1,747.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	96.2
ONE TO FIVE YEARS	528.0
FIVE TO TEN YEARS	14.9
OVER TEN YEARS	0.0
Amortized Cost	639.1	713.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	94.6
ONE TO FIVE YEARS	494.6
FIVE TO TEN YEARS	11.9
OVER TEN YEARS	0.0
Fair Value	601.1	667.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	141.6
ONE TO FIVE YEARS	150.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	292.2	403.1
Available for Sale - Fair Value
ONE YEAR OR LESS	140.1
ONE TO FIVE YEARS	140.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	280.4	388.7
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	135.5
ONE TO FIVE YEARS	1,678.3
FIVE TO TEN YEARS	303.1
OVER TEN YEARS	0.0
Amortized Cost	2,116.9	2,530.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	132.6
ONE TO FIVE YEARS	1,582.7
FIVE TO TEN YEARS	260.0
OVER TEN YEARS	0.0
Fair Value	1,975.3	2,371.9
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	405.4
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	405.4	35.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	405.5
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	405.5	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	569.8
ONE TO FIVE YEARS	2,465.3
FIVE TO TEN YEARS	64.1
OVER TEN YEARS	0.0
AMORTIZED COST	3,099.2	2,645.8
Available for Sale - Fair Value
ONE YEAR OR LESS	563.0
ONE TO FIVE YEARS	2,347.1
FIVE TO TEN YEARS	49.7
OVER TEN YEARS	0.0
FAIR VALUE	2,959.8	2,479.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	817.0
ONE TO FIVE YEARS	4,604.4
FIVE TO TEN YEARS	22.4
OVER TEN YEARS	0.0
Amortized Cost	5,443.8	5,703.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	810.2
ONE TO FIVE YEARS	4,216.8
FIVE TO TEN YEARS	17.8
OVER TEN YEARS	0.0
Fair Value	5,044.8	5,268.2
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	731.5
ONE TO FIVE YEARS	3,985.4
FIVE TO TEN YEARS	248.9
OVER TEN YEARS	0.3
AMORTIZED COST	4,966.1	5,544.3
Available for Sale - Fair Value
ONE YEAR OR LESS	711.3
ONE TO FIVE YEARS	3,815.4
FIVE TO TEN YEARS	244.5
OVER TEN YEARS	0.3
FAIR VALUE	4,771.5	5,256.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	43.8
ONE TO FIVE YEARS	93.0
FIVE TO TEN YEARS	109.8
OVER TEN YEARS	0.0
Amortized Cost	246.6	263.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	43.7
ONE TO FIVE YEARS	93.2
FIVE TO TEN YEARS	110.5
OVER TEN YEARS	0.0
Fair Value	247.4	262.7
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	34.9
ONE TO FIVE YEARS	338.9
FIVE TO TEN YEARS	29.8
OVER TEN YEARS	178.0
Amortized Cost	581.6	532.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	33.6
ONE TO FIVE YEARS	292.8
FIVE TO TEN YEARS	21.0
OVER TEN YEARS	51.9
Fair Value	399.3	375.2
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	38.8
ONE TO FIVE YEARS	765.8
FIVE TO TEN YEARS	144.3
OVER TEN YEARS	0.0
AMORTIZED COST	948.9	1,456.9
Available for Sale - Fair Value
ONE YEAR OR LESS	36.7
ONE TO FIVE YEARS	729.2
FIVE TO TEN YEARS	118.4
OVER TEN YEARS	0.0
FAIR VALUE	884.3	$ 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
OVER TEN YEARS	0.0
Amortized Cost	37.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	36.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	$ 36.9