Securities (Securities with Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	$ 26,661.4	$ 22,981.2
Less than 12 months, unrealized losses	293.6	186.9
12 months or longer fair value	3,884.6	4,580.8
12 months or longer unrealized losses	70.4	62.5
Total fair value	30,546.0	27,562.0
Total unrealized losses	364.0	249.4
U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	3,632.7	3,595.0
Less than 12 months, unrealized losses	52.1	32.1
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	3,632.7	3,595.0
Total unrealized losses	52.1	32.1
Obligations of States and Political Subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	550.8	687.8
Less than 12 months, unrealized losses	3.0	3.3
12 months or longer fair value	21.2	52.0
12 months or longer unrealized losses	0.2	0.3
Total fair value	572.0	739.8
Total unrealized losses	3.2	3.6
Government Sponsored Agency
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	9,427.3	6,495.6
Less than 12 months, unrealized losses	113.9	81.3
12 months or longer fair value	2,751.5	2,998.9
12 months or longer unrealized losses	34.0	27.3
Total fair value	12,178.8	9,494.5
Total unrealized losses	147.9	108.6
Non-U.S. Government
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	5,319.3	5,181.8
Less than 12 months, unrealized losses	18.7	7.9
12 months or longer fair value	0.0	0.0
12 months or longer unrealized losses	0.0	0.0
Total fair value	5,319.3	5,181.8
Total unrealized losses	18.7	7.9
Corporate Debt
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,396.6	1,547.3
Less than 12 months, unrealized losses	15.5	9.3
12 months or longer fair value	549.3	922.3
12 months or longer unrealized losses	14.1	14.0
Total fair value	1,945.9	2,469.6
Total unrealized losses	29.6	23.3
Covered Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,476.3	967.5
Less than 12 months, unrealized losses	17.0	7.2
12 months or longer fair value	89.6	89.1
12 months or longer unrealized losses	0.4	0.9
Total fair value	1,565.9	1,056.6
Total unrealized losses	17.4	8.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	2,628.3	1,692.4
Less than 12 months, unrealized losses	22.7	7.5
12 months or longer fair value	186.0	235.8
12 months or longer unrealized losses	0.2	0.4
Total fair value	2,814.3	1,928.2
Total unrealized losses	22.9	7.9
Other Asset-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1,947.5	2,453.7
Less than 12 months, unrealized losses	16.7	8.3
12 months or longer fair value	29.9	29.9
12 months or longer unrealized losses	0.2	0.1
Total fair value	1,977.4	2,483.6
Total unrealized losses	16.9	8.4
Certificates of Deposit
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	0.0	43.7
Less than 12 months, unrealized losses	0.0	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	0.0	43.7
Total unrealized losses	0.0	0.1
Auction Rate
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	1.1	0.0
Less than 12 months, unrealized losses	0.1	0.0
12 months or longer fair value	0.0	3.1
12 months or longer unrealized losses	0.0	0.1
Total fair value	1.1	3.1
Total unrealized losses	0.1	0.1
Commercial Mortgage-Backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	191.3	233.5
Less than 12 months, unrealized losses	3.7	2.6
12 months or longer fair value	209.9	201.6
12 months or longer unrealized losses	2.4	1.5
Total fair value	401.2	435.1
Total unrealized losses	6.1	4.1
Other
Gain (Loss) on Investments [Line Items]
Less than 12 months, fair value	90.2	82.9
Less than 12 months, unrealized losses	30.2	27.3
12 months or longer fair value	47.2	48.1
12 months or longer unrealized losses	18.9	17.9
Total fair value	137.4	131.0
Total unrealized losses	$ 49.1	$ 45.2