Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value
Less than 12 months, fair value	$ 11,611.0	$ 18,648.8
12 months or longer fair value	17,262.4	15,563.4
Total fair value	28,873.4	34,212.2
Unrealized Losses
Less than 12 months, unrealized losses	63.6	83.9
12 months or longer unrealized losses	205.3	273.2
Total unrealized losses	268.9	357.1
U.S. Government
Fair Value
Less than 12 months, fair value	0.0	0.0
12 months or longer fair value	2,674.5	2,862.0
Total fair value	2,674.5	2,862.0
Unrealized Losses
Less than 12 months, unrealized losses	0.0	0.0
12 months or longer unrealized losses	27.0	39.6
Total unrealized losses	27.0	39.6
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	66.2	169.6
12 months or longer fair value	209.8	279.6
Total fair value	276.0	449.2
Unrealized Losses
Less than 12 months, unrealized losses	1.2	2.4
12 months or longer unrealized losses	0.5	1.3
Total unrealized losses	1.7	3.7
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	6,933.4	8,368.8
12 months or longer fair value	8,371.6	6,822.4
Total fair value	15,305.0	15,191.2
Unrealized Losses
Less than 12 months, unrealized losses	31.9	33.5
12 months or longer unrealized losses	92.7	117.0
Total unrealized losses	124.6	150.5
Non-U.S. Government
Fair Value
Less than 12 months, fair value	2,321.8	5,065.2
12 months or longer fair value	1,259.6	1,274.0
Total fair value	3,581.4	6,339.2
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.8
12 months or longer unrealized losses	5.5	9.0
Total unrealized losses	5.6	9.8
Corporate Debt
Fair Value
Less than 12 months, fair value	339.3	712.7
12 months or longer fair value	1,009.1	1,097.4
Total fair value	1,348.4	1,810.1
Unrealized Losses
Less than 12 months, unrealized losses	0.9	4.1
12 months or longer unrealized losses	13.5	18.8
Total unrealized losses	14.4	22.9
Covered Bonds
Fair Value
Less than 12 months, fair value	181.0	646.4
12 months or longer fair value	1,022.9	696.9
Total fair value	1,203.9	1,343.3
Unrealized Losses
Less than 12 months, unrealized losses	0.3	3.7
12 months or longer unrealized losses	7.1	10.4
Total unrealized losses	7.4	14.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	253.4	1,105.0
12 months or longer fair value	1,501.8	1,189.2
Total fair value	1,755.2	2,294.2
Unrealized Losses
Less than 12 months, unrealized losses	0.6	4.6
12 months or longer unrealized losses	6.2	11.2
Total unrealized losses	6.8	15.8
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,436.5	2,507.8
12 months or longer fair value	893.8	954.9
Total fair value	2,330.3	3,462.7
Unrealized Losses
Less than 12 months, unrealized losses	8.7	15.9
12 months or longer unrealized losses	6.1	11.0
Total unrealized losses	14.8	26.9
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	18.7	22.8
12 months or longer fair value	193.5	274.4
Total fair value	212.2	297.2
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.1
12 months or longer unrealized losses	1.9	4.1
Total unrealized losses	2.0	4.2
Other
Fair Value
Less than 12 months, fair value	60.7	50.5
12 months or longer fair value	125.8	112.6
Total fair value	186.5	163.1
Unrealized Losses
Less than 12 months, unrealized losses	19.8	18.8
12 months or longer unrealized losses	44.8	50.8
Total unrealized losses	$ 64.6	$ 69.6