Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 3,580.8
ONE TO FIVE YEARS	14,894.2
FIVE TO TEN YEARS	6,364.3
OVER TEN YEARS	1,102.6
AMORTIZED COST	25,941.9	$ 23,659.0
Available for Sale - Fair Value
ONE YEAR OR LESS	3,542.3
ONE TO FIVE YEARS	14,718.3
FIVE TO TEN YEARS	6,302.4
OVER TEN YEARS	1,075.9
FAIR VALUE	25,638.9	23,089.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	4,875.7
ONE TO FIVE YEARS	11,941.7
FIVE TO TEN YEARS	4,564.0
OVER TEN YEARS	2,116.5
Amortized Cost	23,497.9	26,221.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	4,732.5
ONE TO FIVE YEARS	11,050.2
FIVE TO TEN YEARS	4,113.8
OVER TEN YEARS	1,744.9
Fair Value	21,641.4	24,473.0
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	348.1
ONE TO FIVE YEARS	3,422.0
FIVE TO TEN YEARS	2,351.5
OVER TEN YEARS	0.0
AMORTIZED COST	6,121.6	3,681.5
Available for Sale - Fair Value
ONE YEAR OR LESS	344.2
ONE TO FIVE YEARS	3,412.9
FIVE TO TEN YEARS	2,355.2
OVER TEN YEARS	0.0
FAIR VALUE	6,112.3	3,622.2
Held to Maturity - Amortized Cost
Amortized Cost		0.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	111.2
FIVE TO TEN YEARS	198.4
OVER TEN YEARS	0.0
AMORTIZED COST	309.6	315.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	106.5
FIVE TO TEN YEARS	188.5
OVER TEN YEARS	0.0
FAIR VALUE	295.0	295.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	60.4
ONE TO FIVE YEARS	1,206.8
FIVE TO TEN YEARS	1,039.9
OVER TEN YEARS	264.5
Amortized Cost	2,571.6	2,563.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	60.0
ONE TO FIVE YEARS	1,174.1
FIVE TO TEN YEARS	998.1
OVER TEN YEARS	250.2
Fair Value	2,482.4	2,492.0
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,631.6
ONE TO FIVE YEARS	5,460.5
FIVE TO TEN YEARS	3,409.8
OVER TEN YEARS	1,000.0
AMORTIZED COST	12,501.9	11,744.3
Available for Sale - Fair Value
ONE YEAR OR LESS	2,606.9
ONE TO FIVE YEARS	5,396.1
FIVE TO TEN YEARS	3,375.2
OVER TEN YEARS	973.2
FAIR VALUE	12,351.4	11,553.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	990.3
ONE TO FIVE YEARS	3,419.8
FIVE TO TEN YEARS	3,138.9
OVER TEN YEARS	1,674.3
Amortized Cost	9,223.3	9,355.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	883.6
ONE TO FIVE YEARS	3,038.5
FIVE TO TEN YEARS	2,768.6
OVER TEN YEARS	1,435.0
Fair Value	8,125.7	8,345.2
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	64.8
ONE TO FIVE YEARS	260.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	325.0	284.8
Available for Sale - Fair Value
ONE YEAR OR LESS	63.5
ONE TO FIVE YEARS	244.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	307.6	264.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,569.9
ONE TO FIVE YEARS	1,139.3
FIVE TO TEN YEARS	32.9
OVER TEN YEARS	0.0
Amortized Cost	2,742.1	4,789.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,564.7
ONE TO FIVE YEARS	1,056.5
FIVE TO TEN YEARS	28.7
OVER TEN YEARS	0.0
Fair Value	2,649.9	4,698.6
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	103.1
ONE TO FIVE YEARS	153.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	256.5	287.5
Available for Sale - Fair Value
ONE YEAR OR LESS	102.3
ONE TO FIVE YEARS	147.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	250.2	279.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	247.8
ONE TO FIVE YEARS	347.3
FIVE TO TEN YEARS	15.2
OVER TEN YEARS	0.0
Amortized Cost	610.3	646.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	243.0
ONE TO FIVE YEARS	329.8
FIVE TO TEN YEARS	12.8
OVER TEN YEARS	0.0
Fair Value	585.6	617.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	66.4
ONE TO FIVE YEARS	236.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	302.7	356.8
Available for Sale - Fair Value
ONE YEAR OR LESS	65.9
ONE TO FIVE YEARS	230.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	296.8	347.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	493.8
ONE TO FIVE YEARS	1,389.9
FIVE TO TEN YEARS	226.6
OVER TEN YEARS	0.0
Amortized Cost	2,110.3	2,208.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	492.7
ONE TO FIVE YEARS	1,310.4
FIVE TO TEN YEARS	204.1
OVER TEN YEARS	0.0
Fair Value	2,007.2	2,100.6
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	403.2
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	403.2	585.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	403.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	403.0	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	126.8
ONE TO FIVE YEARS	2,564.7
FIVE TO TEN YEARS	221.1
OVER TEN YEARS	0.0
AMORTIZED COST	2,912.6	3,013.8
Available for Sale - Fair Value
ONE YEAR OR LESS	125.4
ONE TO FIVE YEARS	2,525.9
FIVE TO TEN YEARS	209.5
OVER TEN YEARS	0.0
FAIR VALUE	2,860.8	2,899.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,058.4
ONE TO FIVE YEARS	3,960.5
FIVE TO TEN YEARS	5.6
OVER TEN YEARS	0.0
Amortized Cost	5,024.5	5,245.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,035.1
ONE TO FIVE YEARS	3,697.1
FIVE TO TEN YEARS	4.5
OVER TEN YEARS	0.0
Fair Value	4,736.7	4,953.8
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	218.3
ONE TO FIVE YEARS	2,108.8
FIVE TO TEN YEARS	122.7
OVER TEN YEARS	102.6
AMORTIZED COST	2,552.4	3,061.0
Available for Sale - Fair Value
ONE YEAR OR LESS	213.5
ONE TO FIVE YEARS	2,083.8
FIVE TO TEN YEARS	123.0
OVER TEN YEARS	102.7
FAIR VALUE	2,523.0	2,962.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1.2
ONE TO FIVE YEARS	120.7
FIVE TO TEN YEARS	71.8
OVER TEN YEARS	0.0
Amortized Cost	193.7	214.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	1.2
ONE TO FIVE YEARS	121.5
FIVE TO TEN YEARS	72.1
OVER TEN YEARS	0.0
Fair Value	194.8	214.4
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	50.7
ONE TO FIVE YEARS	319.8
FIVE TO TEN YEARS	33.1
OVER TEN YEARS	177.7
Amortized Cost	581.3	576.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	49.2
ONE TO FIVE YEARS	285.0
FIVE TO TEN YEARS	24.9
OVER TEN YEARS	59.7
Fair Value	418.8	429.3
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	21.7
ONE TO FIVE YEARS	577.1
FIVE TO TEN YEARS	60.8
OVER TEN YEARS	0.0
AMORTIZED COST	659.6	913.5
Available for Sale - Fair Value
ONE YEAR OR LESS	20.6
ONE TO FIVE YEARS	570.2
FIVE TO TEN YEARS	51.0
OVER TEN YEARS	0.0
FAIR VALUE	641.8	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	37.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	$ 37.3	$ 36.8