Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
FAIR VALUE
LESS THAN 12 MONTHS	$ 9,903.8	$ 2,891.7
12 MONTHS OR LONGER	8,183.2	8,781.2
TOTAL	18,087.0	11,672.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	53.3	5.6
12 MONTHS OR LONGER	150.6	153.1
TOTAL	203.9	158.7
U.S. Governments
FAIR VALUE
LESS THAN 12 MONTHS	451.7	0.0
12 MONTHS OR LONGER	94.9	194.2
TOTAL	546.6	194.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.0
12 MONTHS OR LONGER	4.8	5.1
TOTAL	5.3	5.1
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	311.9	313.1
TOTAL	311.9	313.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	9.1	9.3
TOTAL	9.1	9.3
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	4,712.0	1,288.2
12 MONTHS OR LONGER	6,501.8	6,848.5
TOTAL	11,213.8	8,136.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	23.5	1.7
12 MONTHS OR LONGER	90.7	91.6
TOTAL	114.2	93.3
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	108.9	329.7
12 MONTHS OR LONGER	202.6	197.5
TOTAL	311.5	527.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.7	0.1
12 MONTHS OR LONGER	6.0	6.8
TOTAL	6.7	6.9
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	21.5	21.3
12 MONTHS OR LONGER	44.5	43.1
TOTAL	66.0	64.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.7	0.4
12 MONTHS OR LONGER	0.1	0.3
TOTAL	0.8	0.7
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	81.4	80.0
12 MONTHS OR LONGER	63.9	63.7
TOTAL	145.3	143.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.6	1.0
12 MONTHS OR LONGER	1.1	1.2
TOTAL	2.7	2.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	2,169.3	1,007.7
12 MONTHS OR LONGER	522.6	669.1
TOTAL	2,691.9	1,676.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	22.1	2.3
12 MONTHS OR LONGER	28.0	27.3
TOTAL	50.1	29.6
CLOs
FAIR VALUE
LESS THAN 12 MONTHS	1,912.4	109.9
12 MONTHS OR LONGER	0.0	0.0
TOTAL	1,912.4	109.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	2.2	0.1
12 MONTHS OR LONGER	0.0	0.0
TOTAL	2.2	0.1
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	267.2	0.0
12 MONTHS OR LONGER	226.3	265.4
TOTAL	493.5	265.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.8	0.0
12 MONTHS OR LONGER	1.9	3.0
TOTAL	3.7	3.0
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	179.4	54.9
12 MONTHS OR LONGER	214.7	186.6
TOTAL	394.1	241.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.2	0.0
12 MONTHS OR LONGER	8.9	8.5
TOTAL	$ 9.1	$ 8.5