Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Fair Value
Less than 12 months, fair value	$ 10,294.8	$ 2,995.7
12 months or longer fair value	2,213.8	3,370.9
Total fair value	12,508.6	6,366.6
Unrealized Losses
Less than 12 months, unrealized losses	245.9	15.3
12 months or longer unrealized losses	11.8	16.0
Total unrealized losses	257.7	31.3
U.S. Government
Fair Value
Less than 12 months, fair value	237.6
12 months or longer fair value	0.0
Total fair value	237.6
Unrealized Losses
Less than 12 months, unrealized losses	10.7
12 months or longer unrealized losses	0.0
Total unrealized losses	10.7
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	1,456.8	52.3
12 months or longer fair value	0.0	0.0
Total fair value	1,456.8	52.3
Unrealized Losses
Less than 12 months, unrealized losses	52.5	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	52.5	0.1
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	5,915.6	2,402.3
12 months or longer fair value	2,096.4	2,528.7
Total fair value	8,012.0	4,931.0
Unrealized Losses
Less than 12 months, unrealized losses	148.5	13.6
12 months or longer unrealized losses	11.7	13.3
Total unrealized losses	160.2	26.9
Non-U.S. Government
Fair Value
Less than 12 months, fair value	274.4	90.5
12 months or longer fair value	0.0	0.0
Total fair value	274.4	90.5
Unrealized Losses
Less than 12 months, unrealized losses	6.3	0.7
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	6.3	0.7
Corporate Debt
Fair Value
Less than 12 months, fair value	272.6	66.6
12 months or longer fair value	0.0	0.0
Total fair value	272.6	66.6
Unrealized Losses
Less than 12 months, unrealized losses	2.8	0.1
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	2.8	0.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	981.3	162.8
12 months or longer fair value	49.9	49.9
Total fair value	1,031.2	212.7
Unrealized Losses
Less than 12 months, unrealized losses	19.4	0.5
12 months or longer unrealized losses	0.0	0.1
Total unrealized losses	19.4	0.6
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,035.7	176.8
12 months or longer fair value	60.0	792.3
Total fair value	1,095.7	969.1
Unrealized Losses
Less than 12 months, unrealized losses	4.5	0.2
12 months or longer unrealized losses	0.0	2.6
Total unrealized losses	4.5	2.8
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	120.8	44.4
12 months or longer fair value	7.5	0.0
Total fair value	128.3	44.4
Unrealized Losses
Less than 12 months, unrealized losses	1.2	0.1
12 months or longer unrealized losses	0.1	0.0
Total unrealized losses	$ 1.3	$ 0.1