Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 9,280.0	$ 20,930.9
Less than 12 months, unrealized losses	26.8	85.3
12 months or longer fair value	4,484.3	2,617.7
12 months or longer unrealized losses	47.3	55.2
Total fair value	13,764.3	23,548.6
Total unrealized losses	74.1	140.5
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,101.6	3,888.0
Less than 12 months, unrealized losses	0.9	5.5
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	1,101.6	3,888.0
Total unrealized losses	0.9	5.5
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	631.8	15.1
Less than 12 months, unrealized losses	2.1	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	631.8	15.1
Total unrealized losses	2.1	0.1
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	4,437.8	9,208.5
Less than 12 months, unrealized losses	12.8	38.7
12 months or longer fair value	2,615.3	1,213.6
12 months or longer unrealized losses	10.1	7.8
Total fair value	7,053.1	10,422.1
Total unrealized losses	22.9	46.5
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,102.2	314.3
Less than 12 months, unrealized losses	0.1	0.6
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	1,102.2	314.3
Total unrealized losses	0.1	0.6
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	755.2	2,067.6
Less than 12 months, unrealized losses	1.5	10.3
12 months or longer fair value	1,118.8	1,057.1
12 months or longer unrealized losses	11.4	22.8
Total fair value	1,874.0	3,124.7
Total unrealized losses	12.9	33.1
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	280.4	1,598.4
Less than 12 months, unrealized losses	1.0	6.7
12 months or longer fair value	331.5	10.0
12 months or longer unrealized losses	1.1	0.1
Total fair value	611.9	1,608.4
Total unrealized losses	2.1	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	0.8	2.3
12 months or longer fair value	248.9	109.3
12 months or longer unrealized losses	1.1	0.6
Total fair value	819.2	1,242.2
Total unrealized losses	1.9	2.9
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	0.0	2,122.7
Less than 12 months, unrealized losses	0.0	4.0
12 months or longer fair value	93.2	170.6
12 months or longer unrealized losses	0.2	0.2
Total fair value	93.2	2,293.3
Total unrealized losses	0.2	4.2
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	345.8	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	345.8	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	5.3	6.4
12 months or longer unrealized losses	0.3	0.1
Total fair value	5.3	6.4
Total unrealized losses	0.3	0.1
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	30.8	374.4
Less than 12 months, unrealized losses	0.1	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	30.8	374.4
Total unrealized losses	0.1	3.7
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	24.1	28.7
Less than 12 months, unrealized losses	7.4	13.3
12 months or longer fair value	71.3	50.7
12 months or longer unrealized losses	23.1	23.6
Total fair value	95.4	79.4
Total unrealized losses	$ 30.5	$ 36.9