Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
FAIR VALUE
Less Than 12 Months, Fair Value	$ 2,995.7	$ 7,850.3
12 Months or Longer, Fair Value	3,370.9	10,615.2
Total, Fair Value	6,366.6	18,465.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	15.3	57.0
12 Months or Longer, Unrealized Losses	16.0	51.6
Total, Unrealized Losses	31.3	108.6
U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value		252.2
12 Months or Longer, Fair Value		899.7
Total, Fair Value		1,151.9
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses		2.8
12 Months or Longer, Unrealized Losses		2.3
Total, Unrealized Losses		5.1
Obligations of States and Political Subdivisions
FAIR VALUE
Less Than 12 Months, Fair Value	52.3	902.4
12 Months or Longer, Fair Value	0.0	0.0
Total, Fair Value	52.3	902.4
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.1	13.3
12 Months or Longer, Unrealized Losses	0.0	0.0
Total, Unrealized Losses	0.1	13.3
Government Sponsored Agency
FAIR VALUE
Less Than 12 Months, Fair Value	2,402.3	5,405.0
12 Months or Longer, Fair Value	2,528.7	7,818.4
Total, Fair Value	4,931.0	13,223.4
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	13.6	35.6
12 Months or Longer, Unrealized Losses	13.3	42.5
Total, Unrealized Losses	26.9	78.1
Non-U.S. Government
FAIR VALUE
Less Than 12 Months, Fair Value	90.5
12 Months or Longer, Fair Value	0.0
Total, Fair Value	90.5
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.7
12 Months or Longer, Unrealized Losses	0.0
Total, Unrealized Losses	0.7
Corporate Debt
FAIR VALUE
Less Than 12 Months, Fair Value	66.6	279.3
12 Months or Longer, Fair Value	0.0	492.7
Total, Fair Value	66.6	772.0
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.1	1.1
12 Months or Longer, Unrealized Losses	0.0	2.9
Total, Unrealized Losses	0.1	4.0
Covered Bonds
FAIR VALUE
Less Than 12 Months, Fair Value		138.7
12 Months or Longer, Fair Value		25.0
Total, Fair Value		163.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses		0.7
12 Months or Longer, Unrealized Losses		0.1
Total, Unrealized Losses		0.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
Less Than 12 Months, Fair Value	162.8	217.5
12 Months or Longer, Fair Value	49.9	155.2
Total, Fair Value	212.7	372.7
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.5	1.0
12 Months or Longer, Unrealized Losses	0.1	0.1
Total, Unrealized Losses	0.6	1.1
Other Asset-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	176.8	592.4
12 Months or Longer, Fair Value	792.3	1,164.9
Total, Fair Value	969.1	1,757.3
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.2	1.8
12 Months or Longer, Unrealized Losses	2.6	3.5
Total, Unrealized Losses	2.8	5.3
Commercial Mortgage-Backed
FAIR VALUE
Less Than 12 Months, Fair Value	44.4	62.8
12 Months or Longer, Fair Value	0.0	59.3
Total, Fair Value	44.4	122.1
UNREALIZED LOSSES
Less Than 12 Months, Unrealized Losses	0.1	0.7
12 Months or Longer, Unrealized Losses	0.0	0.2
Total, Unrealized Losses	$ 0.1	$ 0.9