Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 16,312.5	$ 18,412.8
Less than 12 months, unrealized losses	106.3	130.2
12 months or longer fair value	3,685.0	3,627.4
12 months or longer unrealized losses	46.5	45.8
Total fair value	19,997.5	22,040.2
Total unrealized losses	152.8	176.0
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,701.3	1,603.0
Less than 12 months, unrealized losses	13.2	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	1,701.3	1,603.0
Total unrealized losses	13.2	14.3
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	750.1	865.3
Less than 12 months, unrealized losses	1.9	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	750.1	865.3
Total unrealized losses	1.9	5.6
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	7,455.5	8,252.5
Less than 12 months, unrealized losses	51.5	58.5
12 months or longer fair value	2,433.8	2,121.0
12 months or longer unrealized losses	12.1	12.1
Total fair value	9,889.3	10,373.5
Total unrealized losses	63.6	70.6
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,729.3	2,957.1
Less than 12 months, unrealized losses	2.1	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	1,729.3	2,957.1
Total unrealized losses	2.1	4.4
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,555.2	1,601.7
Less than 12 months, unrealized losses	9.6	11.2
12 months or longer fair value	807.3	1,054.4
12 months or longer unrealized losses	13.4	14.0
Total fair value	2,362.5	2,656.1
Total unrealized losses	23.0	25.2
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	829.3	809.0
Less than 12 months, unrealized losses	6.0	8.6
12 months or longer fair value	139.2	138.9
12 months or longer unrealized losses	0.2	0.6
Total fair value	968.5	947.9
Total unrealized losses	6.2	9.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,089.2	1,136.1
Less than 12 months, unrealized losses	3.4	5.7
12 months or longer fair value	249.3	249.1
12 months or longer unrealized losses	0.7	0.9
Total fair value	1,338.5	1,385.2
Total unrealized losses	4.1	6.6
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	714.5	584.3
Less than 12 months, unrealized losses	1.5	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	714.5	584.3
Total unrealized losses	1.5	1.3
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value		81.4
Less than 12 months, unrealized losses		0.1
12 months or longer fair value		0.0
12 months or longer unrealized losses		0.0
Total fair value		81.4
Total unrealized losses		0.1
Auction Rate
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	0.4	0.4
Less than 12 months, unrealized losses	0.1	0.1
12 months or longer fair value	3.8	4.3
12 months or longer unrealized losses	0.2	0.2
Total fair value	4.2	4.7
Total unrealized losses	0.3	0.3
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	439.3	471.5
Less than 12 months, unrealized losses	2.9	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	439.3	471.5
Total unrealized losses	2.9	2.5
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	48.4	50.5
Less than 12 months, unrealized losses	14.1	17.9
12 months or longer fair value	51.6	59.7
12 months or longer unrealized losses	19.9	18.0
Total fair value	100.0	110.2
Total unrealized losses	$ 34.0	$ 35.9