Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
FAIR VALUE
LESS THAN 12 MONTHS	$ 12,448.1	$ 2,891.7
12 MONTHS OR LONGER	6,333.5	8,781.2
TOTAL	18,781.6	11,672.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	61.3	5.6
12 MONTHS OR LONGER	67.6	153.1
TOTAL	128.9	158.7
U.S. Governments
FAIR VALUE
LESS THAN 12 MONTHS	730.2	0.0
12 MONTHS OR LONGER	0.0	194.2
TOTAL	730.2	194.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	2.0	0.0
12 MONTHS OR LONGER	0.0	5.1
TOTAL	2.0	5.1
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS		0.0
12 MONTHS OR LONGER		313.1
TOTAL		313.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS		0.0
12 MONTHS OR LONGER		9.3
TOTAL		9.3
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	7,936.0	1,288.2
12 MONTHS OR LONGER	5,781.6	6,848.5
TOTAL	13,717.6	8,136.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	41.6	1.7
12 MONTHS OR LONGER	63.3	91.6
TOTAL	104.9	93.3
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	240.2	329.7
12 MONTHS OR LONGER	68.2	197.5
TOTAL	308.4	527.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.1
12 MONTHS OR LONGER	0.6	6.8
TOTAL	1.1	6.9
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	39.1	21.3
12 MONTHS OR LONGER	0.0	43.1
TOTAL	39.1	64.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.4
12 MONTHS OR LONGER	0.0	0.3
TOTAL	0.5	0.7
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	36.8	80.0
12 MONTHS OR LONGER	64.2	63.7
TOTAL	101.0	143.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.1	1.0
12 MONTHS OR LONGER	0.8	1.2
TOTAL	1.9	2.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	2,087.1	1,007.7
12 MONTHS OR LONGER	249.8	669.1
TOTAL	2,336.9	1,676.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	9.9	2.3
12 MONTHS OR LONGER	2.2	27.3
TOTAL	12.1	29.6
CLOs
FAIR VALUE
LESS THAN 12 MONTHS	371.3	109.9
12 MONTHS OR LONGER	0.0	0.0
TOTAL	371.3	109.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.3	0.1
12 MONTHS OR LONGER	0.0	0.0
TOTAL	0.3	0.1
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	906.8	0.0
12 MONTHS OR LONGER	47.0	265.4
TOTAL	953.8	265.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	5.2	0.0
12 MONTHS OR LONGER	0.4	3.0
TOTAL	5.6	3.0
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	100.6	54.9
12 MONTHS OR LONGER	122.7	186.6
TOTAL	223.3	241.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.2	0.0
12 MONTHS OR LONGER	0.3	8.5
TOTAL	$ 0.5	$ 8.5