Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value
Less than 12 months, fair value	$ 7,974.2	$ 18,648.8
12 months or longer fair value	16,706.1	15,563.4
Total fair value	24,680.3	34,212.2
Unrealized Losses
Less than 12 months, unrealized losses	48.6	83.9
12 months or longer unrealized losses	121.6	273.2
Total unrealized losses	170.2	357.1
U.S. Government
Fair Value
Less than 12 months, fair value	0.0	0.0
12 months or longer fair value	2,691.5	2,862.0
Total fair value	2,691.5	2,862.0
Unrealized Losses
Less than 12 months, unrealized losses	0.0	0.0
12 months or longer unrealized losses	10.0	39.6
Total unrealized losses	10.0	39.6
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	0.0	169.6
12 months or longer fair value	98.8	279.6
Total fair value	98.8	449.2
Unrealized Losses
Less than 12 months, unrealized losses	0.0	2.4
12 months or longer unrealized losses	0.1	1.3
Total unrealized losses	0.1	3.7
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	4,930.5	8,368.8
12 months or longer fair value	8,559.6	6,822.4
Total fair value	13,490.1	15,191.2
Unrealized Losses
Less than 12 months, unrealized losses	14.8	33.5
12 months or longer unrealized losses	48.9	117.0
Total unrealized losses	63.7	150.5
Non-U.S. Government
Fair Value
Less than 12 months, fair value	1,575.3	5,065.2
12 months or longer fair value	1,231.9	1,274.0
Total fair value	2,807.2	6,339.2
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.8
12 months or longer unrealized losses	4.0	9.0
Total unrealized losses	4.1	9.8
Corporate Debt
Fair Value
Less than 12 months, fair value	98.2	712.7
12 months or longer fair value	793.7	1,097.4
Total fair value	891.9	1,810.1
Unrealized Losses
Less than 12 months, unrealized losses	0.2	4.1
12 months or longer unrealized losses	6.1	18.8
Total unrealized losses	6.3	22.9
Covered Bonds
Fair Value
Less than 12 months, fair value	19.9	646.4
12 months or longer fair value	741.7	696.9
Total fair value	761.6	1,343.3
Unrealized Losses
Less than 12 months, unrealized losses	0.1	3.7
12 months or longer unrealized losses	2.6	10.4
Total unrealized losses	2.7	14.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	168.9	1,105.0
12 months or longer fair value	1,343.8	1,189.2
Total fair value	1,512.7	2,294.2
Unrealized Losses
Less than 12 months, unrealized losses	0.4	4.6
12 months or longer unrealized losses	3.4	11.2
Total unrealized losses	3.8	15.8
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,107.6	2,507.8
12 months or longer fair value	960.4	954.9
Total fair value	2,068.0	3,462.7
Unrealized Losses
Less than 12 months, unrealized losses	4.4	15.9
12 months or longer unrealized losses	2.0	11.0
Total unrealized losses	6.4	26.9
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	0.0	22.8
12 months or longer fair value	154.4	274.4
Total fair value	154.4	297.2
Unrealized Losses
Less than 12 months, unrealized losses	0.0	0.1
12 months or longer unrealized losses	0.9	4.1
Total unrealized losses	0.9	4.2
Other
Fair Value
Less than 12 months, fair value	73.8	50.5
12 months or longer fair value	130.3	112.6
Total fair value	204.1	163.1
Unrealized Losses
Less than 12 months, unrealized losses	28.6	18.8
12 months or longer unrealized losses	43.6	50.8
Total unrealized losses	$ 72.2	$ 69.6