Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 5,778.3
ONE TO FIVE YEARS	13,964.6
FIVE TO TEN YEARS	4,888.1
OVER TEN YEARS	833.4
AMORTIZED COST	25,464.4	$ 27,760.0
Available for Sale - Fair Value
ONE YEAR OR LESS	5,685.0
ONE TO FIVE YEARS	13,435.1
FIVE TO TEN YEARS	4,713.0
OVER TEN YEARS	794.4
FAIR VALUE	24,627.5	26,699.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	4,130.3
ONE TO FIVE YEARS	12,644.6
FIVE TO TEN YEARS	5,534.4
OVER TEN YEARS	2,406.9
Amortized Cost	24,716.2	25,036.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	4,009.3
ONE TO FIVE YEARS	11,669.5
FIVE TO TEN YEARS	5,068.0
OVER TEN YEARS	2,076.3
Fair Value	22,823.1	22,879.3
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	1,086.5
ONE TO FIVE YEARS	1,844.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	2,931.1	2,837.7
Available for Sale - Fair Value
ONE YEAR OR LESS	1,085.1
ONE TO FIVE YEARS	1,774.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	2,859.9	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	0.0
FIVE TO TEN YEARS	298.7
OVER TEN YEARS	19.4
AMORTIZED COST	318.1	817.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	273.6
OVER TEN YEARS	18.3
FAIR VALUE	291.9	787.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	46.3
ONE TO FIVE YEARS	832.4
FIVE TO TEN YEARS	1,270.8
OVER TEN YEARS	424.1
Amortized Cost	2,573.6	2,565.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	45.8
ONE TO FIVE YEARS	813.6
FIVE TO TEN YEARS	1,231.5
OVER TEN YEARS	408.5
Fair Value	2,499.4	2,415.5
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,557.4
ONE TO FIVE YEARS	5,406.0
FIVE TO TEN YEARS	3,035.6
OVER TEN YEARS	687.8
AMORTIZED COST	11,686.8	11,892.5
Available for Sale - Fair Value
ONE YEAR OR LESS	2,520.7
ONE TO FIVE YEARS	5,291.3
FIVE TO TEN YEARS	2,968.7
OVER TEN YEARS	652.7
FAIR VALUE	11,433.4	11,545.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	907.6
ONE TO FIVE YEARS	3,012.3
FIVE TO TEN YEARS	3,546.2
OVER TEN YEARS	1,793.8
Amortized Cost	9,259.9	9,407.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	809.9
ONE TO FIVE YEARS	2,706.1
FIVE TO TEN YEARS	3,203.5
OVER TEN YEARS	1,590.6
Fair Value	8,310.1	8,331.7
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	100.2
ONE TO FIVE YEARS	281.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	382.0	387.6
Available for Sale - Fair Value
ONE YEAR OR LESS	100.1
ONE TO FIVE YEARS	261.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	361.4	360.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,359.7
ONE TO FIVE YEARS	1,379.2
FIVE TO TEN YEARS	33.3
OVER TEN YEARS	0.0
Amortized Cost	2,772.2	3,234.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,354.4
ONE TO FIVE YEARS	1,269.0
FIVE TO TEN YEARS	28.0
OVER TEN YEARS	0.0
Fair Value	2,651.4	3,100.3
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	305.4
ONE TO FIVE YEARS	328.9
FIVE TO TEN YEARS	18.3
OVER TEN YEARS	0.0
AMORTIZED COST	652.6	1,774.3
Available for Sale - Fair Value
ONE YEAR OR LESS	303.5
ONE TO FIVE YEARS	315.0
FIVE TO TEN YEARS	16.0
OVER TEN YEARS	0.0
FAIR VALUE	634.5	1,747.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	62.7
ONE TO FIVE YEARS	594.9
FIVE TO TEN YEARS	15.4
OVER TEN YEARS	0.0
Amortized Cost	673.0	713.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	61.5
ONE TO FIVE YEARS	556.5
FIVE TO TEN YEARS	12.4
OVER TEN YEARS	0.0
Fair Value	630.4	667.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	135.2
ONE TO FIVE YEARS	220.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	356.1	403.1
Available for Sale - Fair Value
ONE YEAR OR LESS	134.7
ONE TO FIVE YEARS	209.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	344.4	388.7
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	171.4
ONE TO FIVE YEARS	1,708.9
FIVE TO TEN YEARS	395.5
OVER TEN YEARS	0.0
Amortized Cost	2,275.8	2,530.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	168.4
ONE TO FIVE YEARS	1,619.9
FIVE TO TEN YEARS	339.0
OVER TEN YEARS	0.0
Fair Value	2,127.3	2,371.9
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	544.7
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	544.7	35.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	544.7
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	544.7	35.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	566.6
ONE TO FIVE YEARS	2,131.9
FIVE TO TEN YEARS	220.4
OVER TEN YEARS	0.0
AMORTIZED COST	2,918.9	2,645.8
Available for Sale - Fair Value
ONE YEAR OR LESS	561.3
ONE TO FIVE YEARS	2,038.3
FIVE TO TEN YEARS	188.1
OVER TEN YEARS	0.0
FAIR VALUE	2,787.7	2,479.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	956.3
ONE TO FIVE YEARS	4,687.9
FIVE TO TEN YEARS	56.1
OVER TEN YEARS	0.0
Amortized Cost	5,700.3	5,703.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	944.9
ONE TO FIVE YEARS	4,314.0
FIVE TO TEN YEARS	46.4
OVER TEN YEARS	0.0
Fair Value	5,305.3	5,268.2
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	989.4
ONE TO FIVE YEARS	2,978.0
FIVE TO TEN YEARS	1,154.0
OVER TEN YEARS	126.2
AMORTIZED COST	5,247.6	5,544.3
Available for Sale - Fair Value
ONE YEAR OR LESS	944.4
ONE TO FIVE YEARS	2,812.9
FIVE TO TEN YEARS	1,129.7
OVER TEN YEARS	123.4
FAIR VALUE	5,010.4	5,256.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	28.6
ONE TO FIVE YEARS	96.4
FIVE TO TEN YEARS	182.4
OVER TEN YEARS	28.3
Amortized Cost	335.7	263.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	28.6
ONE TO FIVE YEARS	96.0
FIVE TO TEN YEARS	181.9
OVER TEN YEARS	28.2
Fair Value	334.7	262.7
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	53.0
ONE TO FIVE YEARS	295.0
FIVE TO TEN YEARS	34.7
OVER TEN YEARS	160.7
Amortized Cost	543.4	532.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	51.1
ONE TO FIVE YEARS	257.6
FIVE TO TEN YEARS	25.3
OVER TEN YEARS	49.0
Fair Value	383.0	375.2
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	37.6
ONE TO FIVE YEARS	772.5
FIVE TO TEN YEARS	161.1
OVER TEN YEARS	0.0
AMORTIZED COST	971.2	1,456.9
Available for Sale - Fair Value
ONE YEAR OR LESS	35.2
ONE TO FIVE YEARS	731.8
FIVE TO TEN YEARS	136.9
OVER TEN YEARS	0.0
FAIR VALUE	903.9	$ 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	36.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	$ 36.8