Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value
Less than 12 months, fair value	$ 6,371.2	$ 18,648.8
12 months or longer fair value	14,838.6	15,563.4
Total fair value	21,209.8	34,212.2
Unrealized Losses
Less than 12 months, unrealized losses	51.2	83.9
12 months or longer unrealized losses	103.7	273.2
Total unrealized losses	154.9	357.1
U.S. Government
Fair Value
Less than 12 months, fair value	98.4	0.0
12 months or longer fair value	2,595.9	2,862.0
Total fair value	2,694.3	2,862.0
Unrealized Losses
Less than 12 months, unrealized losses	0.7	0.0
12 months or longer unrealized losses	5.7	39.6
Total unrealized losses	6.4	39.6
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	217.2	169.6
12 months or longer fair value	45.5	279.6
Total fair value	262.7	449.2
Unrealized Losses
Less than 12 months, unrealized losses	1.9	2.4
12 months or longer unrealized losses	0.1	1.3
Total unrealized losses	2.0	3.7
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	3,914.2	8,368.8
12 months or longer fair value	7,957.2	6,822.4
Total fair value	11,871.4	15,191.2
Unrealized Losses
Less than 12 months, unrealized losses	14.0	33.5
12 months or longer unrealized losses	41.8	117.0
Total unrealized losses	55.8	150.5
Non-U.S. Government
Fair Value
Less than 12 months, fair value	287.3	5,065.2
12 months or longer fair value	901.6	1,274.0
Total fair value	1,188.9	6,339.2
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.8
12 months or longer unrealized losses	1.0	9.0
Total unrealized losses	1.1	9.8
Corporate Debt
Fair Value
Less than 12 months, fair value	397.0	712.7
12 months or longer fair value	663.0	1,097.4
Total fair value	1,060.0	1,810.1
Unrealized Losses
Less than 12 months, unrealized losses	0.9	4.1
12 months or longer unrealized losses	3.7	18.8
Total unrealized losses	4.6	22.9
Covered Bonds
Fair Value
Less than 12 months, fair value	206.4	646.4
12 months or longer fair value	308.0	696.9
Total fair value	514.4	1,343.3
Unrealized Losses
Less than 12 months, unrealized losses	0.1	3.7
12 months or longer unrealized losses	1.2	10.4
Total unrealized losses	1.3	14.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	338.5	1,105.0
12 months or longer fair value	840.5	1,189.2
Total fair value	1,179.0	2,294.2
Unrealized Losses
Less than 12 months, unrealized losses	0.9	4.6
12 months or longer unrealized losses	0.9	11.2
Total unrealized losses	1.8	15.8
Other Asset-Backed
Fair Value
Less than 12 months, fair value	815.1	2,507.8
12 months or longer fair value	1,299.2	954.9
Total fair value	2,114.3	3,462.7
Unrealized Losses
Less than 12 months, unrealized losses	2.8	15.9
12 months or longer unrealized losses	2.8	11.0
Total unrealized losses	5.6	26.9
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	36.6	22.8
12 months or longer fair value	81.8	274.4
Total fair value	118.4	297.2
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.1
12 months or longer unrealized losses	0.2	4.1
Total unrealized losses	0.3	4.2
Other
Fair Value
Less than 12 months, fair value	60.5	50.5
12 months or longer fair value	145.9	112.6
Total fair value	206.4	163.1
Unrealized Losses
Less than 12 months, unrealized losses	29.7	18.8
12 months or longer unrealized losses	46.3	50.8
Total unrealized losses	$ 76.0	$ 69.6