Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 8,066.2	$ 20,930.9
Less than 12 months, unrealized losses	33.6	85.3
12 months or longer fair value	2,898.7	2,617.7
12 months or longer unrealized losses	40.3	55.2
Total fair value	10,964.9	23,548.6
Total unrealized losses	73.9	140.5
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	5,410.2	9,208.5
Less than 12 months, unrealized losses	16.3	38.7
12 months or longer fair value	1,351.7	1,213.6
12 months or longer unrealized losses	7.9	7.8
Total fair value	6,761.9	10,422.1
Total unrealized losses	24.2	46.5
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value		314.3
Less than 12 months, unrealized losses		0.6
12 months or longer fair value		0.0
12 months or longer unrealized losses		0.0
Total fair value		314.3
Total unrealized losses		0.6
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value		3,888.0
Less than 12 months, unrealized losses		5.5
12 months or longer fair value		0.0
12 months or longer unrealized losses		0.0
Total fair value		3,888.0
Total unrealized losses		5.5
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value		15.1
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		15.1
Total unrealized losses		0.1
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	523.5	2,067.6
Less than 12 months, unrealized losses	2.5	10.3
12 months or longer fair value	1,016.1	1,057.1
12 months or longer unrealized losses	8.6	22.8
Total fair value	1,539.6	3,124.7
Total unrealized losses	11.1	33.1
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	847.2	1,598.4
Less than 12 months, unrealized losses	3.1	6.7
12 months or longer fair value	141.2	10.0
12 months or longer unrealized losses	2.6	0.1
Total fair value	988.4	1,608.4
Total unrealized losses	5.7	6.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	570.3	1,132.9
Less than 12 months, unrealized losses	1.1	2.3
12 months or longer fair value	148.9	109.3
12 months or longer unrealized losses	1.1	0.6
Total fair value	719.2	1,242.2
Total unrealized losses	2.2	2.9
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	488.2	2,122.7
Less than 12 months, unrealized losses	0.8	4.0
12 months or longer fair value	185.0	170.6
12 months or longer unrealized losses	0.1	0.2
Total fair value	673.2	2,293.3
Total unrealized losses	0.9	4.2
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	191.0	180.3
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	191.0	180.3
Total unrealized losses	0.1	0.1
Auction Rate
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	0.0	0.0
Less than 12 months, unrealized losses	0.0	0.0
12 months or longer fair value	5.5	6.4
12 months or longer unrealized losses	0.2	0.1
Total fair value	5.5	6.4
Total unrealized losses	0.2	0.1
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value		374.4
Less than 12 months, unrealized losses		3.7
12 months or longer fair value		0.0
12 months or longer unrealized losses		0.0
Total fair value		374.4
Total unrealized losses		3.7
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	35.8	28.7
Less than 12 months, unrealized losses	9.7	13.3
12 months or longer fair value	50.3	50.7
12 months or longer unrealized losses	19.8	23.6
Total fair value	86.1	79.4
Total unrealized losses	$ 29.5	$ 36.9