Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Fair Value
Less than 12 months, fair value	$ 8,319.2	$ 2,995.7
12 months or longer fair value	1,546.9	3,370.9
Total fair value	9,866.1	6,366.6
Unrealized Losses
Less than 12 months, unrealized losses	106.4	15.3
12 months or longer unrealized losses	5.8	16.0
Total unrealized losses	112.2	31.3
U.S. Government
Fair Value
Less than 12 months, fair value	241.3
12 months or longer fair value	0.0
Total fair value	241.3
Unrealized Losses
Less than 12 months, unrealized losses	6.9
12 months or longer unrealized losses	0.0
Total unrealized losses	6.9
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	1,042.2	52.3
12 months or longer fair value	0.0	0.0
Total fair value	1,042.2	52.3
Unrealized Losses
Less than 12 months, unrealized losses	18.5	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	18.5	0.1
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	4,879.1	2,402.3
12 months or longer fair value	1,309.8	2,528.7
Total fair value	6,188.9	4,931.0
Unrealized Losses
Less than 12 months, unrealized losses	59.1	13.6
12 months or longer unrealized losses	5.6	13.3
Total unrealized losses	64.7	26.9
Non-U.S. Government
Fair Value
Less than 12 months, fair value	244.5	90.5
12 months or longer fair value	0.0	0.0
Total fair value	244.5	90.5
Unrealized Losses
Less than 12 months, unrealized losses	5.0	0.7
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	5.0	0.7
Corporate Debt
Fair Value
Less than 12 months, fair value	293.3	66.6
12 months or longer fair value	0.0	0.0
Total fair value	293.3	66.6
Unrealized Losses
Less than 12 months, unrealized losses	1.9	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	1.9	0.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	973.9	162.8
12 months or longer fair value	49.9	49.9
Total fair value	1,023.8	212.7
Unrealized Losses
Less than 12 months, unrealized losses	12.7	0.5
12 months or longer unrealized losses	0.1	0.1
Total unrealized losses	12.8	0.6
Other Asset-Backed
Fair Value
Less than 12 months, fair value	556.0	176.8
12 months or longer fair value	184.9	792.3
Total fair value	740.9	969.1
Unrealized Losses
Less than 12 months, unrealized losses	1.7	0.2
12 months or longer unrealized losses	0.1	2.6
Total unrealized losses	1.8	2.8
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	88.9	44.4
12 months or longer fair value	2.3	0.0
Total fair value	91.2	44.4
Unrealized Losses
Less than 12 months, unrealized losses	0.6	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	$ 0.6	$ 0.1