Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 10,787.2	$ 20,930.9
Less than 12 months, unrealized losses	51.2	85.3
12 months or longer fair value	2,594.5	2,617.7
12 months or longer unrealized losses	43.1	55.2
Total fair value	13,381.7	23,548.6
Total unrealized losses	94.3	140.5
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	6,251.9	9,208.5
Less than 12 months, unrealized losses	21.9	38.7
12 months or longer fair value	1,327.1	1,213.6
12 months or longer unrealized losses	8.2	7.8
Total fair value	7,579.0	10,422.1
Total unrealized losses	30.1	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	97.2	15.1
Less than 12 months, unrealized losses	1.0	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	97.2	15.1
Total unrealized losses	1.0	0.1
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	893.3	2,067.6
Less than 12 months, unrealized losses	5.6	10.3
12 months or longer fair value	1,041.7	1,057.1
12 months or longer unrealized losses	12.2	22.8
Total fair value	1,935.0	3,124.7
Total unrealized losses	17.8	33.1
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,054.8	1,598.4
Less than 12 months, unrealized losses	6.5	6.7
12 months or longer fair value	0.0	10.0
12 months or longer unrealized losses	0.0	0.1
Total fair value	1,054.8	1,608.4
Total unrealized losses	6.5	6.8
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	578.6	1,132.9
Less than 12 months, unrealized losses	2.3	2.3
12 months or longer fair value	109.8	109.3
12 months or longer unrealized losses	0.1	0.6
Total fair value	688.4	1,242.2
Total unrealized losses	2.4	2.9
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,383.1	2,122.7
Less than 12 months, unrealized losses	1.4	4.0
12 months or longer fair value	60.3	170.6
12 months or longer unrealized losses	0.1	0.2
Total fair value	1,443.4	2,293.3
Total unrealized losses	1.5	4.2
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	314.3	180.3
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	314.3	180.3
Total unrealized losses	0.2	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	6.0	6.4
12 months or longer unrealized losses	0.5	0.1
Total fair value	6.0	6.4
Total unrealized losses	0.5	0.1
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	140.5	374.4
Less than 12 months, unrealized losses	0.3	3.7
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	140.5	374.4
Total unrealized losses	0.3	3.7
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	73.5	28.7
Less than 12 months, unrealized losses	12.0	13.3
12 months or longer fair value	49.6	50.7
12 months or longer unrealized losses	22.0	23.6
Total fair value	123.1	79.4
Total unrealized losses	$ 34.0	$ 36.9