Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
FAIR VALUE
Less Than 12 Months, Fair Value	$ 16,660.7	$ 2,995.7
12 Months or Longer, Fair Value	2,037.0	3,370.9
Total, Fair Value	18,697.7	6,366.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	223.6	15.3
12 Months or Longer, Unrealized Losses	25.0	16.0
Total, Unrealized Losses	248.6	31.3
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	239.0
12 Months or Longer, Fair Value	0.0
Total, Fair Value	239.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	9.4
12 Months or Longer, Unrealized Losses	0.0
Total, Unrealized Losses	9.4
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	1,699.5	52.3
12 Months or Longer, Fair Value	31.6	0.0
Total, Fair Value	1,731.1	52.3
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	37.4	0.1
12 Months or Longer, Unrealized Losses	1.2	0.0
Total, Unrealized Losses	38.6	0.1
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	8,207.3	2,402.3
12 Months or Longer, Fair Value	1,821.4	2,528.7
Total, Fair Value	10,028.7	4,931.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	90.2	13.6
12 Months or Longer, Unrealized Losses	20.0	13.3
Total, Unrealized Losses	110.2	26.9
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	230.0	90.5
12 Months or Longer, Fair Value	0.0	0.0
Total, Fair Value	230.0	90.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	9.5	0.7
12 Months or Longer, Unrealized Losses	0.0	0.0
Total, Unrealized Losses	9.5	0.7
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	693.7	66.6
12 Months or Longer, Fair Value	0.0	0.0
Total, Fair Value	693.7	66.6
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	9.7	0.1
12 Months or Longer, Unrealized Losses	0.0	0.0
Total, Unrealized Losses	9.7	0.1
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	92.1
12 Months or Longer, Fair Value	0.0
Total, Fair Value	92.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.9
12 Months or Longer, Unrealized Losses	0.0
Total, Unrealized Losses	0.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	1,116.8	162.8
12 Months or Longer, Fair Value	172.3	49.9
Total, Fair Value	1,289.1	212.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	31.9	0.5
12 Months or Longer, Unrealized Losses	3.5	0.1
Total, Unrealized Losses	35.4	0.6
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	3,815.5	176.8
12 Months or Longer, Fair Value	11.7	792.3
Total, Fair Value	3,827.2	969.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	31.4	0.2
12 Months or Longer, Unrealized Losses	0.3	2.6
Total, Unrealized Losses	31.7	2.8
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	566.8	44.4
12 Months or Longer, Fair Value	0.0	0.0
Total, Fair Value	566.8	44.4
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	3.2	0.1
12 Months or Longer, Unrealized Losses	0.0	0.0
Total, Unrealized Losses	$ 3.2	$ 0.1