Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months, fair value	$ 27,898.7	$ 22,981.2
12 months or longer fair value	4,129.5	4,580.8
Total fair value	32,028.2	27,562.0
Unrealized Losses
Less than 12 months, unrealized losses	296.8	186.9
12 months or longer unrealized losses	86.7	62.5
Total unrealized losses	383.5	249.4
U.S. Government
Fair Value
Less than 12 months, fair value	3,374.3	3,595.0
12 months or longer fair value	199.2	0.0
Total fair value	3,573.5	3,595.0
Unrealized Losses
Less than 12 months, unrealized losses	55.5	32.1
12 months or longer unrealized losses	0.7	0.0
Total unrealized losses	56.2	32.1
Obligations of States and Political Subdivisions
Fair Value
Less than 12 months, fair value	678.0	687.8
12 months or longer fair value	7.6	52.0
Total fair value	685.6	739.8
Unrealized Losses
Less than 12 months, unrealized losses	3.5	3.3
12 months or longer unrealized losses	0.1	0.3
Total unrealized losses	3.6	3.6
Government Sponsored Agency
Fair Value
Less than 12 months, fair value	11,209.7	6,495.6
12 months or longer fair value	2,890.3	2,998.9
Total fair value	14,100.0	9,494.5
Unrealized Losses
Less than 12 months, unrealized losses	125.5	81.3
12 months or longer unrealized losses	42.3	27.3
Total unrealized losses	167.8	108.6
Non-U.S. Government
Fair Value
Less than 12 months, fair value	5,285.9	5,181.8
12 months or longer fair value	0.0	0.0
Total fair value	5,285.9	5,181.8
Unrealized Losses
Less than 12 months, unrealized losses	12.3	7.9
12 months or longer unrealized losses	0.0	0.0
Total unrealized losses	12.3	7.9
Corporate Debt
Fair Value
Less than 12 months, fair value	1,443.7	1,547.3
12 months or longer fair value	458.4	922.3
Total fair value	1,902.1	2,469.6
Unrealized Losses
Less than 12 months, unrealized losses	14.6	9.3
12 months or longer unrealized losses	15.5	14.0
Total unrealized losses	30.1	23.3
Covered Bonds
Fair Value
Less than 12 months, fair value	1,311.0	967.5
12 months or longer fair value	171.4	89.1
Total fair value	1,482.4	1,056.6
Unrealized Losses
Less than 12 months, unrealized losses	13.0	7.2
12 months or longer unrealized losses	3.6	0.9
Total unrealized losses	16.6	8.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Fair Value
Less than 12 months, fair value	2,478.2	1,692.4
12 months or longer fair value	175.2	235.8
Total fair value	2,653.4	1,928.2
Unrealized Losses
Less than 12 months, unrealized losses	19.0	7.5
12 months or longer unrealized losses	1.0	0.4
Total unrealized losses	20.0	7.9
Other Asset-Backed
Fair Value
Less than 12 months, fair value	1,794.6	2,453.7
12 months or longer fair value	0.0	29.9
Total fair value	1,794.6	2,483.6
Unrealized Losses
Less than 12 months, unrealized losses	16.8	8.3
12 months or longer unrealized losses	0.0	0.1
Total unrealized losses	16.8	8.4
Certificates of Deposit
Fair Value
Less than 12 months, fair value		43.7
12 months or longer fair value		0.0
Total fair value		43.7
Unrealized Losses
Less than 12 months, unrealized losses		0.1
12 months or longer unrealized losses		0.0
Total unrealized losses		0.1
Auction Rate
Fair Value
Less than 12 months, fair value	1.1	0.0
12 months or longer fair value	0.0	3.1
Total fair value	1.1	3.1
Unrealized Losses
Less than 12 months, unrealized losses	0.1	0.0
12 months or longer unrealized losses	0.0	0.1
Total unrealized losses	0.1	0.1
Commercial Mortgage-Backed
Fair Value
Less than 12 months, fair value	228.3	233.5
12 months or longer fair value	181.8	201.6
Total fair value	410.1	435.1
Unrealized Losses
Less than 12 months, unrealized losses	4.2	2.6
12 months or longer unrealized losses	2.0	1.5
Total unrealized losses	6.2	4.1
Other
Fair Value
Less than 12 months, fair value	93.9	82.9
12 months or longer fair value	45.6	48.1
Total fair value	139.5	131.0
Unrealized Losses
Less than 12 months, unrealized losses	32.3	27.3
12 months or longer unrealized losses	21.5	17.9
Total unrealized losses	$ 53.8	$ 45.2