Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value
Less than 12 months, fair value	$ 10,590.2	$ 2,995.7
12 months or longer fair value	1,312.7	3,370.9
Total fair value	11,902.9	6,366.6
Unrealized Losses
Less than 12 months, unrealized losses	147.7	15.3
12 months or longer unrealized losses	5.1	16.0
Total unrealized losses	152.8	31.3
U.S. Government
Fair Value
Less than 12 months, fair value	241.8
12 months or longer fair value	0.0
Total fair value	241.8
Unrealized Losses
Less than 12 months, unrealized losses	7.5
12 months or longer unrealized losses	0.0
Total unrealized losses	7.5
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	1,500.0	52.3
12 months or longer fair value	0.0	0.0
Total fair value	1,500.0	52.3
Unrealized Losses
Less than 12 months, unrealized losses	27.3	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	27.3	0.1
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	4,762.1	2,402.3
12 months or longer fair value	1,202.8	2,528.7
Total fair value	5,964.9	4,931.0
Unrealized Losses
Less than 12 months, unrealized losses	85.9	13.6
12 months or longer unrealized losses	5.0	13.3
Total unrealized losses	90.9	26.9
Non-U.S. Government
Fair Value
Less than 12 months, fair value	235.1	90.5
12 months or longer fair value	0.0	0.0
Total fair value	235.1	90.5
Unrealized Losses
Less than 12 months, unrealized losses	4.0	0.7
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	4.0	0.7
Corporate Debt
Fair Value
Less than 12 months, fair value	495.4	66.6
12 months or longer fair value	0.0	0.0
Total fair value	495.4	66.6
Unrealized Losses
Less than 12 months, unrealized losses	2.4	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	2.4	0.1
Covered Bonds
Fair Value
Less than 12 months, fair value	47.7
12 months or longer fair value	0.0
Total fair value	47.7
Unrealized Losses
Less than 12 months, unrealized losses	0.0
12 months or longer unrealized losses	0.0
Total unrealized losses	0.0
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	999.0	162.8
12 months or longer fair value	49.9	49.9
Total fair value	1,048.9	212.7
Unrealized Losses
Less than 12 months, unrealized losses	14.0	0.5
12 months or longer unrealized losses	0.1	0.1
Total unrealized losses	14.1	0.6
Other Asset-Backed
Fair Value
Less than 12 months, fair value	2,141.7	176.8
12 months or longer fair value	60.0	792.3
Total fair value	2,201.7	969.1
Unrealized Losses
Less than 12 months, unrealized losses	5.8	0.2
12 months or longer unrealized losses	0.0	2.6
Total unrealized losses	5.8	2.8
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	167.4	44.4
12 months or longer fair value	0.0	0.0
Total fair value	167.4	44.4
Unrealized Losses
Less than 12 months, unrealized losses	0.8	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	$ 0.8	$ 0.1