Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 3,796.9
ONE TO FIVE YEARS	16,736.5
FIVE TO TEN YEARS	5,676.6
OVER TEN YEARS	917.3
AMORTIZED COST	27,127.3	$ 23,659.0
Available for Sale - Fair Value
ONE YEAR OR LESS	3,768.4
ONE TO FIVE YEARS	16,571.9
FIVE TO TEN YEARS	5,627.3
OVER TEN YEARS	894.1
FAIR VALUE	26,861.7	23,089.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	5,049.6
ONE TO FIVE YEARS	11,267.4
FIVE TO TEN YEARS	4,280.4
OVER TEN YEARS	2,201.2
Amortized Cost	22,798.6	26,221.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	4,913.6
ONE TO FIVE YEARS	10,402.9
FIVE TO TEN YEARS	3,832.3
OVER TEN YEARS	1,793.3
Fair Value	20,942.1	24,473.0
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	448.2
ONE TO FIVE YEARS	3,315.3
FIVE TO TEN YEARS	2,338.6
OVER TEN YEARS	0.0
AMORTIZED COST	6,102.1	3,681.5
Available for Sale - Fair Value
ONE YEAR OR LESS	444.3
ONE TO FIVE YEARS	3,304.3
FIVE TO TEN YEARS	2,341.3
OVER TEN YEARS	0.0
FAIR VALUE	6,089.9	3,622.2
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	175.3
FIVE TO TEN YEARS	133.4
OVER TEN YEARS	0.0
AMORTIZED COST	308.7	315.8
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	167.4
FIVE TO TEN YEARS	126.0
OVER TEN YEARS	0.0
FAIR VALUE	293.4	295.8
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	60.1
ONE TO FIVE YEARS	1,280.1
FIVE TO TEN YEARS	974.6
OVER TEN YEARS	243.1
Amortized Cost	2,557.9	2,563.9
Held to Maturity - Fair Value
ONE YEAR OR LESS	59.5
ONE TO FIVE YEARS	1,241.7
FIVE TO TEN YEARS	929.7
OVER TEN YEARS	228.3
Fair Value	2,459.2	2,492.0
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,681.3
ONE TO FIVE YEARS	6,828.4
FIVE TO TEN YEARS	2,521.5
OVER TEN YEARS	712.7
AMORTIZED COST	12,743.9	11,744.3
Available for Sale - Fair Value
ONE YEAR OR LESS	2,665.6
ONE TO FIVE YEARS	6,783.5
FIVE TO TEN YEARS	2,494.8
OVER TEN YEARS	689.2
FAIR VALUE	12,633.1	11,553.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	919.0
ONE TO FIVE YEARS	3,394.9
FIVE TO TEN YEARS	2,956.1
OVER TEN YEARS	1,771.2
Amortized Cost	9,041.2	9,355.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	821.5
ONE TO FIVE YEARS	3,004.0
FIVE TO TEN YEARS	2,590.2
OVER TEN YEARS	1,504.5
Fair Value	7,920.2	8,345.2
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	66.2
ONE TO FIVE YEARS	263.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	329.3	284.8
Available for Sale - Fair Value
ONE YEAR OR LESS	65.3
ONE TO FIVE YEARS	245.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	311.2	264.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,436.5
ONE TO FIVE YEARS	1,034.1
FIVE TO TEN YEARS	32.7
OVER TEN YEARS	0.0
Amortized Cost	2,503.3	4,789.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,431.1
ONE TO FIVE YEARS	956.7
FIVE TO TEN YEARS	28.3
OVER TEN YEARS	0.0
Fair Value	2,416.1	4,698.6
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	129.9
ONE TO FIVE YEARS	95.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	225.6	287.5
Available for Sale - Fair Value
ONE YEAR OR LESS	129.0
ONE TO FIVE YEARS	90.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	219.7	279.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	279.6
ONE TO FIVE YEARS	259.0
FIVE TO TEN YEARS	15.1
OVER TEN YEARS	0.0
Amortized Cost	553.7	646.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	274.3
ONE TO FIVE YEARS	245.8
FIVE TO TEN YEARS	12.7
OVER TEN YEARS	0.0
Fair Value	532.8	617.9
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	67.6
ONE TO FIVE YEARS	237.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	304.7	356.8
Available for Sale - Fair Value
ONE YEAR OR LESS	67.5
ONE TO FIVE YEARS	232.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	299.9	347.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	605.6
ONE TO FIVE YEARS	1,203.0
FIVE TO TEN YEARS	214.9
OVER TEN YEARS	0.0
Amortized Cost	2,023.5	2,208.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	600.4
ONE TO FIVE YEARS	1,132.7
FIVE TO TEN YEARS	194.3
OVER TEN YEARS	0.0
Fair Value	1,927.4	2,100.6
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	425.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	425.0	585.1
Held to Maturity - Fair Value
ONE YEAR OR LESS	424.8
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	424.8	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	168.9
ONE TO FIVE YEARS	3,343.5
FIVE TO TEN YEARS	298.1
OVER TEN YEARS	0.0
AMORTIZED COST	3,810.5	3,013.8
Available for Sale - Fair Value
ONE YEAR OR LESS	166.8
ONE TO FIVE YEARS	3,300.7
FIVE TO TEN YEARS	285.9
OVER TEN YEARS	0.0
FAIR VALUE	3,753.4	2,899.9
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,241.4
ONE TO FIVE YEARS	3,698.5
FIVE TO TEN YEARS	5.6
OVER TEN YEARS	0.0
Amortized Cost	4,945.5	5,245.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,222.0
ONE TO FIVE YEARS	3,456.3
FIVE TO TEN YEARS	4.4
OVER TEN YEARS	0.0
Fair Value	4,682.7	4,953.8
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	212.1
ONE TO FIVE YEARS	1,879.7
FIVE TO TEN YEARS	350.4
OVER TEN YEARS	204.6
AMORTIZED COST	2,646.8	3,061.0
Available for Sale - Fair Value
ONE YEAR OR LESS	207.9
ONE TO FIVE YEARS	1,861.3
FIVE TO TEN YEARS	351.7
OVER TEN YEARS	204.9
FAIR VALUE	2,625.8	2,962.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	10.9
ONE TO FIVE YEARS	64.0
FIVE TO TEN YEARS	46.3
OVER TEN YEARS	0.0
Amortized Cost	121.2	214.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	10.8
ONE TO FIVE YEARS	64.4
FIVE TO TEN YEARS	46.5
OVER TEN YEARS	0.0
Fair Value	121.7	214.4
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	22.7
ONE TO FIVE YEARS	598.4
FIVE TO TEN YEARS	34.6
OVER TEN YEARS	0.0
AMORTIZED COST	655.7	913.5
Available for Sale - Fair Value
ONE YEAR OR LESS	22.0
ONE TO FIVE YEARS	585.7
FIVE TO TEN YEARS	27.6
OVER TEN YEARS	0.0
FAIR VALUE	635.3	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	36.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	36.9	36.8
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	71.5
ONE TO FIVE YEARS	296.2
FIVE TO TEN YEARS	35.1
OVER TEN YEARS	186.9
Amortized Cost	589.7	576.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	69.2
ONE TO FIVE YEARS	264.4
FIVE TO TEN YEARS	26.2
OVER TEN YEARS	60.5
Fair Value	$ 420.3	$ 429.3