Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	$ 9,524.2	$ 7,101.7
Less than 12 months, unrealized losses	37.4	26.5
12 months or longer fair value	2,177.0	3,309.1
12 months or longer unrealized losses	37.4	61.1
Total fair value	11,701.2	10,410.8
Total unrealized losses	74.8	87.6
Government Sponsored Agency
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	4,502.5	2,344.9
Less than 12 months, unrealized losses	7.8	6.6
12 months or longer fair value	701.0	1,730.0
12 months or longer unrealized losses	4.9	12.9
Total fair value	5,203.5	4,074.9
Total unrealized losses	12.7	19.5
U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value		998.2
Less than 12 months, unrealized losses		1.7
12 months or longer fair value		0.0
12 months or longer unrealized losses		0.0
Total fair value		998.2
Total unrealized losses		1.7
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	132.3	292.9
Less than 12 months, unrealized losses	0.4	0.4
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	132.3	292.9
Total unrealized losses	0.4	0.4
Corporate Debt
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	1,244.0	1,244.5
Less than 12 months, unrealized losses	6.0	3.9
12 months or longer fair value	1,022.0	1,338.8
12 months or longer unrealized losses	14.1	37.7
Total fair value	2,266.0	2,583.3
Total unrealized losses	20.1	41.6
Covered Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	842.0	142.3
Less than 12 months, unrealized losses	3.6	0.2
12 months or longer fair value	0.0	10.0
12 months or longer unrealized losses	0.0	0.1
Total fair value	842.0	152.3
Total unrealized losses	3.6	0.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	494.3
Less than 12 months, unrealized losses	0.4
12 months or longer fair value	223.3
12 months or longer unrealized losses	0.1
Total fair value	717.6
Total unrealized losses	0.5
Supranational and Non-U.S. Agency Bonds
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value		313.2
Less than 12 months, unrealized losses		0.3
12 months or longer fair value		175.5
12 months or longer unrealized losses		1.1
Total fair value		488.7
Total unrealized losses		1.4
Residential Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value		0.0
Less than 12 months, unrealized losses		0.0
12 months or longer fair value		4.5
12 months or longer unrealized losses		0.5
Total fair value		4.5
Total unrealized losses		0.5
Other Asset-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	1,657.6	1,297.6
Less than 12 months, unrealized losses	2.0	1.0
12 months or longer fair value	179.5	0.0
12 months or longer unrealized losses	0.1	0.0
Total fair value	1,837.1	1,297.6
Total unrealized losses	2.1	1.0
Non-U.S. Government
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	555.1	438.6
Less than 12 months, unrealized losses	0.2	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	555.1	438.6
Total unrealized losses	0.2	0.1
Auction Rate
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	1.4	2.4
Less than 12 months, unrealized losses	0.2	0.2
12 months or longer fair value	4.3	4.7
12 months or longer unrealized losses	0.8	0.6
Total fair value	5.7	7.1
Total unrealized losses	1.0	0.8
Commercial Mortgage-Backed
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	57.7
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	57.7
Total unrealized losses	0.1
Other
Investments, Unrealized Loss Position [Line Items]
Less than 12 months, fair value	37.3	27.1
Less than 12 months, unrealized losses	16.7	12.1
12 months or longer fair value	46.9	45.6
12 months or longer unrealized losses	17.4	8.2
Total fair value	84.2	72.7
Total unrealized losses	$ 34.1	$ 20.3