Securities (Securities with Unrealized Losses) (Details) $ in Millions	Sep. 30, 2025 USD ($)	Dec. 31, 2024 USD ($) security
FAIR VALUE
LESS THAN 12 MONTHS	$ 4,667.4	$ 8,748.5
12 MONTHS OR LONGER	9,502.9	9,083.7
TOTAL	14,170.3	17,832.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	8.6	28.3
12 MONTHS OR LONGER	160.2	226.8
TOTAL	168.8	255.1
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	2,261.8	4,477.5
12 MONTHS OR LONGER	896.7	532.3
TOTAL	3,158.5	5,009.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.6	11.1
12 MONTHS OR LONGER	6.9	11.8
TOTAL	8.5	22.9
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	310.7	297.5
TOTAL	310.7	297.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	11.3	13.6
TOTAL	11.3	13.6
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	2,012.4	3,298.8
12 MONTHS OR LONGER	6,540.1	6,373.8
TOTAL	8,552.5	9,672.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	6.7	14.5
12 MONTHS OR LONGER	92.7	118.0
TOTAL	99.4	132.5
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	0.0	54.9
12 MONTHS OR LONGER	195.3	181.6
TOTAL	195.3	236.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.1
12 MONTHS OR LONGER	7.3	12.2
TOTAL	7.3	12.3
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	21.3	0.0
12 MONTHS OR LONGER	85.6	76.9
TOTAL	106.9	76.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.1	0.0
12 MONTHS OR LONGER	0.7	1.6
TOTAL	0.8	$ 1.6
Number of debt securities, available-for-sale | security		1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss		$ 38.9
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss		1.3
Allowance for credit losses for AFS securities		0.2
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	36.1	0.0
12 MONTHS OR LONGER	63.3	119.6
TOTAL	99.4	119.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.2	0.0
12 MONTHS OR LONGER	1.6	3.6
TOTAL	1.8	3.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	117.7	881.9
12 MONTHS OR LONGER	878.7	720.5
TOTAL	996.4	1,602.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	2.6
12 MONTHS OR LONGER	26.4	39.6
TOTAL	26.4	42.2
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	105.0	35.4
12 MONTHS OR LONGER	273.0	405.4
TOTAL	378.0	440.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	4.5	12.9
TOTAL	4.5	12.9
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	113.1	0.0
12 MONTHS OR LONGER	259.5	376.1
TOTAL	372.6	376.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	8.8	13.5
TOTAL	$ 8.8	$ 13.5