Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total	$ 35,407.5	$ 33,864.8
Gross Unrealized Gains	71.9	66.6
Gross Unrealized Losses	294.9	189.3
Fair Value	35,184.5	33,742.1
U.S. Government
Investment Holdings [Line Items]
Total	5,668.7	5,714.4
Gross Unrealized Gains	26.0	18.0
Gross Unrealized Losses	56.2	32.1
Fair Value	5,638.5	5,700.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	741.3	749.9
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	3.6	3.5
Fair Value	737.8	746.4
Government Sponsored Agency
Investment Holdings [Line Items]
Total	21,186.9	18,745.3
Gross Unrealized Gains	35.4	39.9
Gross Unrealized Losses	167.8	108.6
Fair Value	21,054.5	18,676.6
Non-U.S. Government
Investment Holdings [Line Items]
Total	143.2	179.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	2.0	1.9
Fair Value	141.2	177.2
Corporate Debt
Investment Holdings [Line Items]
Total	2,493.1	3,013.7
Gross Unrealized Gains	2.6	2.2
Gross Unrealized Losses	29.5	22.9
Fair Value	2,466.2	2,993.0
Covered Bonds
Investment Holdings [Line Items]
Total	838.0	879.0
Gross Unrealized Gains	0.9	1.0
Gross Unrealized Losses	7.0	4.4
Fair Value	831.9	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,008.7	1,819.8
Gross Unrealized Gains	6.2	4.0
Gross Unrealized Losses	5.9	3.8
Fair Value	2,009.0	1,820.0
Other Asset-Backed
Investment Holdings [Line Items]
Total	1,871.7	2,297.7
Gross Unrealized Gains	0.7	1.5
Gross Unrealized Losses	16.6	7.9
Fair Value	1,855.8	2,291.3
Auction Rate
Investment Holdings [Line Items]
Total	2.9	4.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.1	0.1
Fair Value	2.8	4.3
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	433.9	439.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	6.2	4.1
Fair Value	427.7	435.1
Other
Investment Holdings [Line Items]
Total	19.1	22.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 19.1	$ 22.3