Securities (Securities with Unrealized Losses) (Details) $ in Millions	Dec. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security
FAIR VALUE
Less Than 12 Months, Fair Value	$ 8,748.5	$ 503.0
12 Months or Longer, Fair Value	9,083.7	18,932.3
Total, Fair Value	17,832.2	19,435.3
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	28.3	24.1
12 Months or Longer, Unrealized Losses	226.8	544.2
Total, Unrealized Losses	255.1	568.3
Allowance for credit losses, available-for-sale securities	0.2	1.2
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	4,477.5	0.0
12 Months or Longer, Fair Value	532.3	3,364.7
Total, Fair Value	5,009.8	3,364.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	11.1	0.0
12 Months or Longer, Unrealized Losses	11.8	61.5
Total, Unrealized Losses	22.9	61.5
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	87.8
12 Months or Longer, Fair Value	297.5	208.0
Total, Fair Value	297.5	295.8
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	5.9
12 Months or Longer, Unrealized Losses	13.6	14.1
Total, Unrealized Losses	13.6	20.0
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	3,298.8	331.0
12 Months or Longer, Fair Value	6,373.8	9,486.6
Total, Fair Value	9,672.6	9,817.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	14.5	11.5
12 Months or Longer, Unrealized Losses	118.0	188.8
Total, Unrealized Losses	132.5	200.3
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	54.9	0.0
12 Months or Longer, Fair Value	181.6	264.5
Total, Fair Value	236.5	264.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.1	0.0
12 Months or Longer, Unrealized Losses	12.2	20.4
Total, Unrealized Losses	12.3	20.4
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	4.4
12 Months or Longer, Fair Value	76.9	143.0
Total, Fair Value	76.9	147.4
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	0.1
12 Months or Longer, Unrealized Losses	1.6	2.1
Total, Unrealized Losses	$ 1.6	$ 2.2
Number of debt securities, available-for-sale | security	1	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 38.9	$ 98.4
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	1.3	5.9
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	0.0
12 Months or Longer, Fair Value	119.6	213.2
Total, Fair Value	119.6	213.2
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	0.0
12 Months or Longer, Unrealized Losses	3.6	9.7
Total, Unrealized Losses	3.6	9.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	881.9	0.0
12 Months or Longer, Fair Value	720.5	2,477.0
Total, Fair Value	1,602.4	2,477.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	2.6	0.0
12 Months or Longer, Unrealized Losses	39.6	105.8
Total, Unrealized Losses	42.2	$ 105.8
Number of debt securities, available-for-sale | security		1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss		$ 71.0
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss		8.2
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	35.4	19.8
12 Months or Longer, Fair Value	405.4	1,998.7
Total, Fair Value	440.8	2,018.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	2.0
12 Months or Longer, Unrealized Losses	12.9	98.0
Total, Unrealized Losses	12.9	100.0
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	60.0
12 Months or Longer, Fair Value	376.1	776.6
Total, Fair Value	376.1	836.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	4.6
12 Months or Longer, Unrealized Losses	13.5	43.8
Total, Unrealized Losses	$ 13.5	$ 48.4