Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 3,340.5
ONE TO FIVE YEARS	10,163.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	38,198.3	$ 34,102.4
Available for Sale - Fair Value
ONE YEAR OR LESS	3,337.8
ONE TO FIVE YEARS	10,171.5
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	38,123.1	34,036.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	6,846.0
ONE TO FIVE YEARS	7,380.4
FIVE TO TEN YEARS	858.8
OVER TEN YEARS	209.4
AMORTIZED COST	23,492.1	23,429.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	6,815.4
ONE TO FIVE YEARS	7,244.0
FIVE TO TEN YEARS	828.1
OVER TEN YEARS	73.3
FAIR VALUE	22,353.5	22,381.2
Amortized Cost
ONE YEAR OR LESS	10,186.5
ONE TO FIVE YEARS	17,543.9
FIVE TO TEN YEARS	858.8
OVER TEN YEARS	209.4
TOTAL	61,690.4	57,532.0
Fair Value
ONE YEAR OR LESS	10,153.2
ONE TO FIVE YEARS	17,415.5
FIVE TO TEN YEARS	828.1
OVER TEN YEARS	73.3
TOTAL	60,476.6	56,417.7
U.S. Governments
Available for Sale - Amortized Cost
ONE YEAR OR LESS	1,696.8
ONE TO FIVE YEARS	6,175.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	7,872.2
AMORTIZED COST	7,872.2	8,148.0
Available for Sale - Fair Value
ONE YEAR OR LESS	1,698.3
ONE TO FIVE YEARS	6,185.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	7,883.3
FAIR VALUE	7,883.3	8,172.4
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
AMORTIZED COST		322.4
Available for Sale - Fair Value
FAIR VALUE		313.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	303.5
ONE TO FIVE YEARS	1,332.9
FIVE TO TEN YEARS	726.6
OVER TEN YEARS	21.8
AMORTIZED COST	2,384.8
AMORTIZED COST	2,384.8	2,457.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	302.8
ONE TO FIVE YEARS	1,318.0
FIVE TO TEN YEARS	705.8
OVER TEN YEARS	20.9
FAIR VALUE	2,347.5
FAIR VALUE	2,347.5	2,449.4
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	241.8
ONE TO FIVE YEARS	167.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	409.0
AMORTIZED COST	409.0	534.1
Available for Sale - Fair Value
ONE YEAR OR LESS	241.1
ONE TO FIVE YEARS	167.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	408.5
FAIR VALUE	408.5	527.2
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	3,413.5
ONE TO FIVE YEARS	1,147.7
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	4,561.2
AMORTIZED COST	4,561.2	4,741.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	3,406.3
ONE TO FIVE YEARS	1,134.3
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	4,540.6
FAIR VALUE	4,540.6	4,713.9
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	271.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	271.2
AMORTIZED COST	271.2	65.1
Available for Sale - Fair Value
ONE YEAR OR LESS	0.0
ONE TO FIVE YEARS	271.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	271.4
FAIR VALUE	271.4	64.4
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	109.7
ONE TO FIVE YEARS	170.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	280.3
AMORTIZED COST	280.3	389.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	108.5
ONE TO FIVE YEARS	167.6
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	276.1
FAIR VALUE	276.1	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	36.8
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	275.7
FAIR VALUE	275.7	273.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	682.5
ONE TO FIVE YEARS	1,432.4
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	2,114.9
AMORTIZED COST	2,114.9	1,754.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	677.6
ONE TO FIVE YEARS	1,397.1
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	2,074.7
FAIR VALUE	2,074.7	1,713.2
Certificate of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	751.8
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	751.8
AMORTIZED COST	751.8	444.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	751.8
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	751.8
FAIR VALUE	751.8	440.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	1,162.3
ONE TO FIVE YEARS	3,511.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	4,674.2
AMORTIZED COST	4,674.2	5,002.9
Available for Sale - Fair Value
ONE YEAR OR LESS	1,159.5
ONE TO FIVE YEARS	3,510.9
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	4,670.4
FAIR VALUE	4,670.4	4,984.3
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,508.7
ONE TO FIVE YEARS	2,997.3
FIVE TO TEN YEARS	89.4
OVER TEN YEARS	0.0
AMORTIZED COST	4,595.4
AMORTIZED COST	4,595.4	4,511.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,493.4
ONE TO FIVE YEARS	2,956.6
FIVE TO TEN YEARS	89.6
OVER TEN YEARS	0.0
FAIR VALUE	4,539.6
FAIR VALUE	4,539.6	4,456.4
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	76.3
ONE TO FIVE YEARS	299.5
FIVE TO TEN YEARS	42.8
OVER TEN YEARS	187.6
AMORTIZED COST	606.2
AMORTIZED COST	606.2	669.2
Held to Maturity - Fair Value
ONE YEAR OR LESS	75.0
ONE TO FIVE YEARS	270.4
FIVE TO TEN YEARS	32.7
OVER TEN YEARS	52.4
FAIR VALUE	430.5
FAIR VALUE	430.5	491.4
Government Sponsored Agency
Available for Sale - Amortized Cost
AMORTIZED COST	19,381.1
AMORTIZED COST	19,381.1	16,616.7
Available for Sale - Fair Value
FAIR VALUE	19,302.8
FAIR VALUE	19,302.8	16,567.5
Held to Maturity - Amortized Cost
AMORTIZED COST	8,112.8
AMORTIZED COST	8,112.8	8,424.5
Held to Maturity - Fair Value
FAIR VALUE	7,309.8
FAIR VALUE	7,309.8	7,696.1
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
AMORTIZED COST	511.7
AMORTIZED COST	511.7	417.4
Available for Sale - Fair Value
FAIR VALUE	511.6
FAIR VALUE	511.6	409.0
Held to Maturity - Amortized Cost
AMORTIZED COST	37.6
AMORTIZED COST	37.6	37.6
Held to Maturity - Fair Value
FAIR VALUE	35.8
FAIR VALUE	35.8	36.3
CLOs
Available for Sale - Amortized Cost
AMORTIZED COST	3,451.7
AMORTIZED COST	3,451.7	2,151.3
Available for Sale - Fair Value
FAIR VALUE	3,453.7
FAIR VALUE	3,453.7	2,154.9
Other Asset-Backed
Available for Sale - Amortized Cost
AMORTIZED COST	1,349.8
AMORTIZED COST	1,349.8	569.2
Available for Sale - Fair Value
FAIR VALUE	1,345.7
FAIR VALUE	1,345.7	$ 570.2
Held to Maturity - Amortized Cost
AMORTIZED COST	47.1
AMORTIZED COST	47.1
Held to Maturity - Fair Value
FAIR VALUE	47.1
FAIR VALUE	$ 47.1