Securities (Securities with Unrealized Losses) (Details) $ in Millions	Dec. 31, 2023 USD ($) Security	Dec. 31, 2022 USD ($) Security
FAIR VALUE
Less Than 12 Months, Fair Value	$ 503.0	$ 15,665.4
12 Months or Longer, Fair Value	18,932.3	5,769.4
Total, Fair Value	19,435.3	21,434.8
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	24.1	727.7
12 Months or Longer, Unrealized Losses	544.2	321.5
Total, Unrealized Losses	568.3	1,049.2
Allowance for credit losses, available-for-sale securities	1.2	1.3
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	1,123.6
12 Months or Longer, Fair Value	3,364.7	343.1
Total, Fair Value	3,364.7	1,466.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	64.1
12 Months or Longer, Unrealized Losses	61.5	28.7
Total, Unrealized Losses	61.5	92.8
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	87.8	160.0
12 Months or Longer, Fair Value	208.0	120.6
Total, Fair Value	295.8	280.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	5.9	16.8
12 Months or Longer, Unrealized Losses	14.1	13.4
Total, Unrealized Losses	20.0	30.2
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	331.0	7,631.4
12 Months or Longer, Fair Value	9,486.6	2,737.7
Total, Fair Value	9,817.6	10,369.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	11.5	262.1
12 Months or Longer, Unrealized Losses	188.8	89.5
Total, Unrealized Losses	200.3	351.6
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	235.4
12 Months or Longer, Fair Value	264.5	124.6
Total, Fair Value	264.5	360.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	17.3
12 Months or Longer, Unrealized Losses	20.4	10.3
Total, Unrealized Losses	20.4	27.6
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	4.4	427.3
12 Months or Longer, Fair Value	143.0	130.3
Total, Fair Value	147.4	557.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.1	14.6
12 Months or Longer, Unrealized Losses	2.1	2.8
Total, Unrealized Losses	$ 2.2	$ 17.4
Number of debt securities, available-for-sale | Security	3	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 98.4	$ 93.8
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	5.9	9.5
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	238.0
12 Months or Longer, Fair Value	213.2	42.8
Total, Fair Value	213.2	280.8
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	13.5
12 Months or Longer, Unrealized Losses	9.7	1.2
Total, Unrealized Losses	9.7	14.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	0.0	1,305.4
12 Months or Longer, Fair Value	2,477.0	807.1
Total, Fair Value	2,477.0	2,112.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.0	74.4
12 Months or Longer, Unrealized Losses	105.8	83.2
Total, Unrealized Losses	$ 105.8	$ 157.6
Number of debt securities, available-for-sale | Security	1	1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 71.0	$ 68.3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	8.2	9.1
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	19.8	3,873.4
12 Months or Longer, Fair Value	1,998.7	1,247.6
Total, Fair Value	2,018.5	5,121.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	2.0	217.5
12 Months or Longer, Unrealized Losses	98.0	70.6
Total, Unrealized Losses	100.0	288.1
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	60.0	670.9
12 Months or Longer, Fair Value	776.6	215.6
Total, Fair Value	836.6	886.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	4.6	47.4
12 Months or Longer, Unrealized Losses	43.8	21.8
Total, Unrealized Losses	$ 48.4	$ 69.2