Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value
Less than 12 months, fair value	$ 6,022.3	$ 7,850.3
12 months or longer fair value	8,409.6	10,615.2
Total fair value	14,431.9	18,465.5
Unrealized Losses
Less than 12 months, unrealized losses	95.5	57.0
12 months or longer unrealized losses	144.5	51.6
Total unrealized losses	240.0	108.6
Debt securities, available-for-sale, unrealized loss position, accumulated loss, excluded for credit losses	0.6
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	230.6	902.4
12 months or longer fair value	0.0	0.0
Total fair value	230.6	902.4
Unrealized Losses
Less than 12 months, unrealized losses	3.2	13.3
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	3.2	13.3
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	2,803.1	5,405.0
12 months or longer fair value	7,033.0	7,818.4
Total fair value	9,836.1	13,223.4
Unrealized Losses
Less than 12 months, unrealized losses	24.4	35.6
12 months or longer unrealized losses	81.9	42.5
Total unrealized losses	106.3	78.1
Corporate Debt
Fair Value
Less than 12 months, fair value	669.2	279.3
12 months or longer fair value	134.0	492.7
Total fair value	803.2	772.0
Unrealized Losses
Less than 12 months, unrealized losses	8.8	1.1
12 months or longer unrealized losses	1.9	2.9
Total unrealized losses	10.7	4.0
Debt securities, available-for-sale, continuous unrealized loss position, excluded for credit losses	24.6
Covered Bonds
Fair Value
Less than 12 months, fair value	306.5	138.7
12 months or longer fair value	0.0	25.0
Total fair value	306.5	163.7
Unrealized Losses
Less than 12 months, unrealized losses	1.3	0.7
12 months or longer unrealized losses	0.0	0.1
Total unrealized losses	1.3	0.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	99.9	217.5
12 months or longer fair value	158.8	155.2
Total fair value	258.7	372.7
Unrealized Losses
Less than 12 months, unrealized losses	0.1	1.0
12 months or longer unrealized losses	0.5	0.1
Total unrealized losses	0.6	1.1
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,616.7	592.4
12 months or longer fair value	1,049.7	1,164.9
Total fair value	2,666.4	1,757.3
Unrealized Losses
Less than 12 months, unrealized losses	45.0	1.8
12 months or longer unrealized losses	60.1	3.5
Total unrealized losses	105.1	5.3
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	296.3	62.8
12 months or longer fair value	34.1	59.3
Total fair value	330.4	122.1
Unrealized Losses
Less than 12 months, unrealized losses	12.7	0.7
12 months or longer unrealized losses	0.1	0.2
Total unrealized losses	$ 12.8	$ 0.9