Securities (Securities with Unrealized Losses) (Details) $ in Millions	Dec. 31, 2022 USD ($) Security	Dec. 31, 2021 USD ($)
FAIR VALUE
Less Than 12 Months, Fair Value	$ 15,665.4	$ 16,660.7
12 Months or Longer, Fair Value	5,769.4	2,037.0
Total, Fair Value	21,434.8	18,697.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	727.7	223.6
12 Months or Longer, Unrealized Losses	321.5	25.0
Total, Unrealized Losses	1,049.2	248.6
Allowance for credit losses, available-for-sale securities	567.2	531.9
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	1,123.6	239.0
12 Months or Longer, Fair Value	343.1	0.0
Total, Fair Value	1,466.7	239.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	64.1	9.4
12 Months or Longer, Unrealized Losses	28.7	0.0
Total, Unrealized Losses	92.8	9.4
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	160.0	1,699.5
12 Months or Longer, Fair Value	120.6	31.6
Total, Fair Value	280.6	1,731.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	16.8	37.4
12 Months or Longer, Unrealized Losses	13.4	1.2
Total, Unrealized Losses	30.2	38.6
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	7,631.4	8,207.3
12 Months or Longer, Fair Value	2,737.7	1,821.4
Total, Fair Value	10,369.1	10,028.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	262.1	90.2
12 Months or Longer, Unrealized Losses	89.5	20.0
Total, Unrealized Losses	351.6	110.2
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	235.4	230.0
12 Months or Longer, Fair Value	124.6	0.0
Total, Fair Value	360.0	230.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	17.3	9.5
12 Months or Longer, Unrealized Losses	10.3	0.0
Total, Unrealized Losses	27.6	9.5
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	427.3	693.7
12 Months or Longer, Fair Value	130.3	0.0
Total, Fair Value	557.6	693.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	14.6	9.7
12 Months or Longer, Unrealized Losses	2.8	0.0
Total, Unrealized Losses	$ 17.4	9.7
Number of debt securities, available-for-sale | Security	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 93.8
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	9.5
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	238.0	92.1
12 Months or Longer, Fair Value	42.8	0.0
Total, Fair Value	280.8	92.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	13.5	0.9
12 Months or Longer, Unrealized Losses	1.2	0.0
Total, Unrealized Losses	14.7	0.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	1,305.4	1,116.8
12 Months or Longer, Fair Value	807.1	172.3
Total, Fair Value	2,112.5	1,289.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	74.4	31.9
12 Months or Longer, Unrealized Losses	83.2	3.5
Total, Unrealized Losses	$ 157.6	35.4
Number of debt securities, available-for-sale | Security	1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 68.3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	9.1
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	3,873.4	3,815.5
12 Months or Longer, Fair Value	1,247.6	11.7
Total, Fair Value	5,121.0	3,827.2
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	217.5	31.4
12 Months or Longer, Unrealized Losses	70.6	0.3
Total, Unrealized Losses	288.1	31.7
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	670.9	566.8
12 Months or Longer, Fair Value	215.6	0.0
Total, Fair Value	886.5	566.8
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	47.4	3.2
12 Months or Longer, Unrealized Losses	21.8	0.0
Total, Unrealized Losses	69.2	$ 3.2
Debt Securities And Sub-Sovereign, Supranational and Non US Agency Bonds
UNREALIZED LOSSES
Allowance for credit losses, available-for-sale securities	$ 1.3