Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 3,255.1
ONE TO FIVE YEARS	10,601.4
FIVE TO TEN YEARS	149.4
OVER TEN YEARS	0.0
AMORTIZED COST	37,065.7	$ 34,102.4
Available for Sale - Fair Value
ONE YEAR OR LESS	3,253.6
ONE TO FIVE YEARS	10,550.2
FIVE TO TEN YEARS	147.4
OVER TEN YEARS	0.0
FAIR VALUE	36,913.2	34,036.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	7,409.1
ONE TO FIVE YEARS	7,507.6
FIVE TO TEN YEARS	815.4
OVER TEN YEARS	216.9
AMORTIZED COST	24,286.2	23,429.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	7,360.9
ONE TO FIVE YEARS	7,344.2
FIVE TO TEN YEARS	792.1
OVER TEN YEARS	76.2
FAIR VALUE	23,138.0	22,381.2
Amortized Cost
ONE YEAR OR LESS	10,664.2
ONE TO FIVE YEARS	18,109.0
FIVE TO TEN YEARS	964.8
OVER TEN YEARS	216.9
TOTAL	61,351.9	57,532.0
Fair Value
ONE YEAR OR LESS	10,614.5
ONE TO FIVE YEARS	17,894.4
FIVE TO TEN YEARS	939.5
OVER TEN YEARS	76.2
TOTAL	60,051.2	56,417.7
U.S. Governments
Available for Sale - Amortized Cost
ONE YEAR OR LESS	1,695.2
ONE TO FIVE YEARS	6,270.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	7,965.3
AMORTIZED COST	7,965.3	8,148.0
Available for Sale - Fair Value
ONE YEAR OR LESS	1,699.7
ONE TO FIVE YEARS	6,272.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	7,971.9
FAIR VALUE	7,971.9	8,172.4
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	248.9
FIVE TO TEN YEARS	72.1
OVER TEN YEARS	0.0
AMORTIZED COST	321.0
AMORTIZED COST	321.0	322.4
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	242.3
FIVE TO TEN YEARS	69.6
OVER TEN YEARS	0.0
FAIR VALUE	311.9
FAIR VALUE	311.9	313.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	231.7
ONE TO FIVE YEARS	1,412.6
FIVE TO TEN YEARS	772.3
OVER TEN YEARS	25.5
AMORTIZED COST	2,442.1
AMORTIZED COST	2,442.1	2,457.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	231.5
ONE TO FIVE YEARS	1,407.3
FIVE TO TEN YEARS	757.4
OVER TEN YEARS	24.7
FAIR VALUE	2,420.9
FAIR VALUE	2,420.9	2,449.4
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	179.8
ONE TO FIVE YEARS	138.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	318.3
AMORTIZED COST	318.3	534.1
Available for Sale - Fair Value
ONE YEAR OR LESS	178.4
ONE TO FIVE YEARS	133.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	311.6
FAIR VALUE	311.6	527.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	3,989.6
ONE TO FIVE YEARS	1,037.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	5,026.8
AMORTIZED COST	5,026.8	4,741.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	3,978.5
ONE TO FIVE YEARS	1,015.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	4,994.0
FAIR VALUE	4,994.0	4,713.9
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	44.6
ONE TO FIVE YEARS	22.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	66.9
AMORTIZED COST	66.9	65.1
Available for Sale - Fair Value
ONE YEAR OR LESS	44.5
ONE TO FIVE YEARS	21.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	66.1
FAIR VALUE	66.1	64.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	131.5
ONE TO FIVE YEARS	172.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	303.5
AMORTIZED COST	303.5	389.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	129.4
ONE TO FIVE YEARS	168.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	297.5
FAIR VALUE	297.5	384.0
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	239.6
ONE TO FIVE YEARS	37.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	277.4
AMORTIZED COST	277.4	275.3
Available for Sale - Fair Value
ONE YEAR OR LESS	238.9
ONE TO FIVE YEARS	36.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	275.0
FAIR VALUE	275.0	273.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	752.4
ONE TO FIVE YEARS	1,440.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	2,192.8
AMORTIZED COST	2,192.8	1,754.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	744.7
ONE TO FIVE YEARS	1,395.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	2,140.0
FAIR VALUE	2,140.0	1,713.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	1,095.9
ONE TO FIVE YEARS	3,883.8
FIVE TO TEN YEARS	77.3
OVER TEN YEARS	0.0
AMORTIZED COST	5,057.0
AMORTIZED COST	5,057.0	5,002.9
Available for Sale - Fair Value
ONE YEAR OR LESS	1,092.1
ONE TO FIVE YEARS	3,844.8
FIVE TO TEN YEARS	77.8
OVER TEN YEARS	0.0
FAIR VALUE	5,014.7
FAIR VALUE	5,014.7	4,984.3
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,449.9
ONE TO FIVE YEARS	3,155.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	4,604.9
AMORTIZED COST	4,604.9	4,511.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,428.5
ONE TO FIVE YEARS	3,099.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	4,527.7
FAIR VALUE	4,527.7	4,456.4
Government Sponsored Agency
Available for Sale - Amortized Cost
AMORTIZED COST	18,614.1
AMORTIZED COST	18,614.1	16,616.7
Available for Sale - Fair Value
FAIR VALUE	18,529.1
FAIR VALUE	18,529.1	16,567.5
Held to Maturity - Amortized Cost
AMORTIZED COST	8,299.6
AMORTIZED COST	8,299.6	8,424.5
Held to Maturity - Fair Value
FAIR VALUE	7,528.3
FAIR VALUE	7,528.3	7,696.1
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
AMORTIZED COST	445.2
AMORTIZED COST	445.2	417.4
Available for Sale - Fair Value
FAIR VALUE	436.1
FAIR VALUE	436.1	409.0
Held to Maturity - Amortized Cost
AMORTIZED COST	37.6
AMORTIZED COST	37.6	37.6
Held to Maturity - Fair Value
FAIR VALUE	36.3
FAIR VALUE	36.3	36.3
CLOs
Available for Sale - Amortized Cost
AMORTIZED COST	3,295.2
AMORTIZED COST	3,295.2	2,151.3
Available for Sale - Fair Value
FAIR VALUE	3,293.4
FAIR VALUE	3,293.4	2,154.9
Other Asset-Backed
Available for Sale - Amortized Cost
AMORTIZED COST	705.3
AMORTIZED COST	705.3	569.2
Available for Sale - Fair Value
FAIR VALUE	703.4
FAIR VALUE	703.4	570.2
Certificate of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	766.5
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	766.5
AMORTIZED COST	766.5	444.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	762.8
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	762.8
FAIR VALUE	762.8	440.5
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	87.5
ONE TO FIVE YEARS	290.4
FIVE TO TEN YEARS	43.1
OVER TEN YEARS	191.4
AMORTIZED COST	612.4
AMORTIZED COST	612.4	669.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	85.5
ONE TO FIVE YEARS	258.8
FIVE TO TEN YEARS	34.7
OVER TEN YEARS	51.5
FAIR VALUE	430.5
FAIR VALUE	$ 430.5	$ 491.4