Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value
Less than 12 months, fair value	$ 3,758.4	$ 7,850.3
12 months or longer fair value	3,943.4	10,615.2
Total fair value	7,701.8	18,465.5
Unrealized Losses
Less than 12 months, unrealized losses	10.8	57.0
12 months or longer unrealized losses	24.2	51.6
Total unrealized losses	35.0	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	230.4	902.4
12 months or longer fair value	0.0	0.0
Total fair value	230.4	902.4
Unrealized Losses
Less than 12 months, unrealized losses	1.0	13.3
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	1.0	13.3
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	2,578.6	5,405.0
12 months or longer fair value	2,862.3	7,818.4
Total fair value	5,440.9	13,223.4
Unrealized Losses
Less than 12 months, unrealized losses	6.8	35.6
12 months or longer unrealized losses	14.4	42.5
Total unrealized losses	21.2	78.1
Non-U.S. Government
Fair Value
Less than 12 months, fair value	26.5
12 months or longer fair value	0.0
Total fair value	26.5
Unrealized Losses
Less than 12 months, unrealized losses	0.1
12 months or longer unrealized losses	0.0
Total unrealized losses	0.1
Corporate Debt
Fair Value
Less than 12 months, fair value	69.1	279.3
12 months or longer fair value	76.1	492.7
Total fair value	145.2	772.0
Unrealized Losses
Less than 12 months, unrealized losses	1.0	1.1
12 months or longer unrealized losses	1.2	2.9
Total unrealized losses	2.2	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	146.3	217.5
12 months or longer fair value	158.7	155.2
Total fair value	305.0	372.7
Unrealized Losses
Less than 12 months, unrealized losses	0.2	1.0
12 months or longer unrealized losses	0.1	0.1
Total unrealized losses	0.3	1.1
Other Asset-Backed
Fair Value
Less than 12 months, fair value	631.2	592.4
12 months or longer fair value	846.3	1,164.9
Total fair value	1,477.5	1,757.3
Unrealized Losses
Less than 12 months, unrealized losses	1.0	1.8
12 months or longer unrealized losses	8.5	3.5
Total unrealized losses	9.5	5.3
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	76.3	62.8
12 months or longer fair value	0.0	59.3
Total fair value	76.3	122.1
Unrealized Losses
Less than 12 months, unrealized losses	0.7	0.7
12 months or longer unrealized losses	0.0	0.2
Total unrealized losses	$ 0.7	$ 0.9