Securities (Securities with Unrealized Losses) (Details) $ in Millions	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($) security
FAIR VALUE
LESS THAN 12 MONTHS	$ 5,419.0	$ 8,748.5
12 MONTHS OR LONGER	8,798.9	9,083.7
TOTAL	14,217.9	17,832.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	13.1	28.3
12 MONTHS OR LONGER	193.8	226.8
TOTAL	206.9	255.1
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	1,105.8	4,477.5
12 MONTHS OR LONGER	582.4	532.3
TOTAL	1,688.2	5,009.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.5	11.1
12 MONTHS OR LONGER	9.4	11.8
TOTAL	10.9	22.9
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	303.4	297.5
TOTAL	303.4	297.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	13.9	13.6
TOTAL	13.9	13.6
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	3,350.3	3,298.8
12 MONTHS OR LONGER	6,193.3	6,373.8
TOTAL	9,543.6	9,672.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	9.7	14.5
12 MONTHS OR LONGER	100.3	118.0
TOTAL	110.0	132.5
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	0.0	54.9
12 MONTHS OR LONGER	184.1	181.6
TOTAL	184.1	236.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.1
12 MONTHS OR LONGER	10.2	12.2
TOTAL	10.2	12.3
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	90.9	76.9
TOTAL	90.9	76.9
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	2.0	1.6
TOTAL	2.0	$ 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
Net amount of recognized liabilities for repurchase agreements	0.0	0.2
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	120.8	119.6
TOTAL	120.8	119.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	2.8	3.6
TOTAL	2.8	3.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	443.4	881.9
12 MONTHS OR LONGER	673.0	720.5
TOTAL	1,116.4	1,602.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.9	2.6
12 MONTHS OR LONGER	33.8	39.6
TOTAL	34.7	42.2
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	319.4	35.4
12 MONTHS OR LONGER	323.8	405.4
TOTAL	643.2	440.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.0
12 MONTHS OR LONGER	9.1	12.9
TOTAL	9.6	12.9
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	200.1	0.0
12 MONTHS OR LONGER	327.2	376.1
TOTAL	527.3	376.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.0
12 MONTHS OR LONGER	12.3	13.5
TOTAL	$ 12.8	$ 13.5