Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 19,325.6	$ 18,412.8
Less than 12 months, unrealized losses	110.3	130.2
12 months or longer fair value	4,216.0	3,627.4
12 months or longer unrealized losses	49.4	45.8
Total fair value	23,541.6	22,040.2
Total unrealized losses	159.7	176.0
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	2,109.7	1,603.0
Less than 12 months, unrealized losses	9.4	14.3
12 months or longer fair value	99.9	0.0
12 months or longer unrealized losses	2.0	0.0
Total fair value	2,209.6	1,603.0
Total unrealized losses	11.4	14.3
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	586.9	865.3
Less than 12 months, unrealized losses	0.7	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	586.9	865.3
Total unrealized losses	0.7	5.6
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	7,032.9	8,252.5
Less than 12 months, unrealized losses	46.4	58.5
12 months or longer fair value	2,808.3	2,121.0
12 months or longer unrealized losses	15.3	12.1
Total fair value	9,841.2	10,373.5
Total unrealized losses	61.7	70.6
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	2,972.5	2,957.1
Less than 12 months, unrealized losses	9.3	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	2,972.5	2,957.1
Total unrealized losses	9.3	4.4
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,229.9	1,601.7
Less than 12 months, unrealized losses	5.2	11.2
12 months or longer fair value	941.7	1,054.4
12 months or longer unrealized losses	11.4	14.0
Total fair value	2,171.6	2,656.1
Total unrealized losses	16.6	25.2
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,069.8	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	1,069.8	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	2,350.6	1,136.1
Less than 12 months, unrealized losses	9.8	5.7
12 months or longer fair value	309.2	249.1
12 months or longer unrealized losses	0.8	0.9
Total fair value	2,659.8	1,385.2
Total unrealized losses	10.6	6.6
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,347.3	584.3
Less than 12 months, unrealized losses	2.8	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	1,347.3	584.3
Total unrealized losses	2.8	1.3
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	61.1	81.4
Less than 12 months, unrealized losses	0.1	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	61.1	81.4
Total unrealized losses	0.1	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	466.5	471.5
Less than 12 months, unrealized losses	2.5	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	466.5	471.5
Total unrealized losses	2.5	2.5
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	98.4	50.5
Less than 12 months, unrealized losses	17.7	17.9
12 months or longer fair value	53.2	59.7
12 months or longer unrealized losses	19.6	18.0
Total fair value	151.6	110.2
Total unrealized losses	$ 37.3	$ 35.9