Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Amortized Cost	$ 34,186.3	$ 35,631.9
Gross unrealized, gains	87.4	78.7
Gross unrealized, losses	105.9	130.8
Fair Value	34,167.8	35,579.8
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	6,032.6	7,514.5
Gross unrealized, gains	23.9	22.4
Gross unrealized, losses	11.7	14.3
Fair Value	6,044.8	7,522.6
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	834.4	890.8
Gross unrealized, gains	0.2	0.0
Gross unrealized, losses	1.4	5.6
Fair Value	833.2	885.2
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	17,796.1	17,914.1
Gross unrealized, gains	50.5	49.3
Gross unrealized, losses	61.6	70.6
Fair Value	17,785.0	17,892.8
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	139.8	420.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	1.2	2.1
Fair Value	138.6	417.9
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,326.3	3,787.4
Gross unrealized, gains	3.4	2.6
Gross unrealized, losses	19.7	24.8
Fair Value	3,310.0	3,765.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	837.9	1,148.6
Gross unrealized, gains	1.0	0.8
Gross unrealized, losses	3.3	5.5
Fair Value	835.6	1,143.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,901.4	1,343.6
Gross unrealized, gains	3.9	0.9
Gross unrealized, losses	2.8	3.8
Fair Value	1,902.5	1,340.7
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,829.8	2,083.7
Gross unrealized, gains	4.5	2.7
Gross unrealized, losses	1.7	1.3
Fair Value	2,832.6	2,085.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	4.4	5.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.3	0.3
Fair Value	4.1	4.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	445.6	474.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	2.2	2.5
Fair Value	443.4	471.6
Other
Investment Holdings [Line Items]
Amortized Cost	38.0	50.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.0	0.0
Fair Value	$ 38.0	$ 50.1