Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 17,163.2	$ 18,412.8
Less than 12 months, unrealized losses	107.0	130.2
12 months or longer fair value	3,893.4	3,627.4
12 months or longer unrealized losses	46.6	45.8
Total fair value	21,056.6	22,040.2
Total unrealized losses	153.6	176.0
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,701.5	1,603.0
Less than 12 months, unrealized losses	11.7	14.3
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	1,701.5	1,603.0
Total unrealized losses	11.7	14.3
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	662.4	865.3
Less than 12 months, unrealized losses	1.4	5.6
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	662.4	865.3
Total unrealized losses	1.4	5.6
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	6,306.7	8,252.5
Less than 12 months, unrealized losses	48.2	58.5
12 months or longer fair value	2,794.3	2,121.0
12 months or longer unrealized losses	13.4	12.1
Total fair value	9,101.0	10,373.5
Total unrealized losses	61.6	70.6
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	2,968.2	2,957.1
Less than 12 months, unrealized losses	5.1	4.4
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	2,968.2	2,957.1
Total unrealized losses	5.1	4.4
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,375.4	1,601.7
Less than 12 months, unrealized losses	8.1	11.2
12 months or longer fair value	796.7	1,054.4
12 months or longer unrealized losses	11.8	14.0
Total fair value	2,172.1	2,656.1
Total unrealized losses	19.9	25.2
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	994.1	809.0
Less than 12 months, unrealized losses	6.4	8.6
12 months or longer fair value	0.0	138.9
12 months or longer unrealized losses	0.0	0.6
Total fair value	994.1	947.9
Total unrealized losses	6.4	9.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,509.8	1,136.1
Less than 12 months, unrealized losses	5.8	5.7
12 months or longer fair value	249.4	249.1
12 months or longer unrealized losses	0.6	0.9
Total fair value	1,759.2	1,385.2
Total unrealized losses	6.4	6.6
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,100.2	584.3
Less than 12 months, unrealized losses	1.7	1.3
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	1,100.2	584.3
Total unrealized losses	1.7	1.3
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	72.3	81.4
Less than 12 months, unrealized losses	0.6	0.1
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	72.3	81.4
Total unrealized losses	0.6	0.1
Auction Rate
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	0.0	0.4
Less than 12 months, unrealized losses	0.0	0.1
12 months or longer fair value	3.7	4.3
12 months or longer unrealized losses	0.3	0.2
Total fair value	3.7	4.7
Total unrealized losses	0.3	0.3
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	416.5	471.5
Less than 12 months, unrealized losses	2.2	2.5
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	416.5	471.5
Total unrealized losses	2.2	2.5
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	56.1	50.5
Less than 12 months, unrealized losses	15.8	17.9
12 months or longer fair value	49.3	59.7
12 months or longer unrealized losses	20.5	18.0
Total fair value	105.4	110.2
Total unrealized losses	$ 36.3	$ 35.9