Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value
Less than 12 months, fair value	$ 2,285.4	$ 7,850.3
12 months or longer fair value	4,283.9	10,615.2
Total fair value	6,569.3	18,465.5
Unrealized Losses
Less than 12 months, unrealized losses	14.6	57.0
12 months or longer unrealized losses	37.1	51.6
Total unrealized losses	51.7	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	115.6	902.4
12 months or longer fair value	0.0	0.0
Total fair value	115.6	902.4
Unrealized Losses
Less than 12 months, unrealized losses	0.4	13.3
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	0.4	13.3
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	1,435.4	5,405.0
12 months or longer fair value	3,200.6	7,818.4
Total fair value	4,636.0	13,223.4
Unrealized Losses
Less than 12 months, unrealized losses	6.5	35.6
12 months or longer unrealized losses	15.4	42.5
Total unrealized losses	21.9	78.1
Corporate Debt
Fair Value
Less than 12 months, fair value	144.4	279.3
12 months or longer fair value	89.5	492.7
Total fair value	233.9	772.0
Unrealized Losses
Less than 12 months, unrealized losses	1.5	1.1
12 months or longer unrealized losses	1.2	2.9
Total unrealized losses	2.7	4.0
Covered Bonds
Fair Value
Less than 12 months, fair value	88.1	138.7
12 months or longer fair value	0.0	25.0
Total fair value	88.1	163.7
Unrealized Losses
Less than 12 months, unrealized losses	0.3	0.7
12 months or longer unrealized losses	0.0	0.1
Total unrealized losses	0.3	0.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	130.9	217.5
12 months or longer fair value	159.2	155.2
Total fair value	290.1	372.7
Unrealized Losses
Less than 12 months, unrealized losses	0.1	1.0
12 months or longer unrealized losses	0.2	0.1
Total unrealized losses	0.3	1.1
Other Asset-Backed
Fair Value
Less than 12 months, fair value	273.8	592.4
12 months or longer fair value	834.6	1,164.9
Total fair value	1,108.4	1,757.3
Unrealized Losses
Less than 12 months, unrealized losses	4.1	1.8
12 months or longer unrealized losses	20.3	3.5
Total unrealized losses	24.4	5.3
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	97.2	62.8
12 months or longer fair value	0.0	59.3
Total fair value	97.2	122.1
Unrealized Losses
Less than 12 months, unrealized losses	1.7	0.7
12 months or longer unrealized losses	0.0	0.2
Total unrealized losses	$ 1.7	$ 0.9