Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Total	$ 36,397.0	$ 33,864.8
Gross Unrealized Gains	61.9	66.6
Gross Unrealized Losses	327.8	189.3
Fair Value	36,131.1	33,742.1
U.S. Government
Investment Holdings [Line Items]
Total	5,662.0	5,714.4
Gross Unrealized Gains	21.6	18.0
Gross Unrealized Losses	58.8	32.1
Fair Value	5,624.8	5,700.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	718.9	749.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	6.6	3.5
Fair Value	712.3	746.4
Government Sponsored Agency
Investment Holdings [Line Items]
Total	21,753.4	18,745.3
Gross Unrealized Gains	29.9	39.9
Gross Unrealized Losses	199.6	108.6
Fair Value	21,583.7	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	1.8	1.9
Fair Value	141.4	177.2
Corporate Debt
Investment Holdings [Line Items]
Total	2,295.5	3,013.7
Gross Unrealized Gains	2.7	2.2
Gross Unrealized Losses	27.0	22.9
Fair Value	2,271.2	2,993.0
Covered Bonds
Investment Holdings [Line Items]
Total	830.9	879.0
Gross Unrealized Gains	0.8	1.0
Gross Unrealized Losses	6.8	4.4
Fair Value	824.9	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	1,920.7	1,819.8
Gross Unrealized Gains	6.0	4.0
Gross Unrealized Losses	5.0	3.8
Fair Value	1,921.7	1,820.0
Other Asset-Backed
Investment Holdings [Line Items]
Total	2,601.8	2,297.7
Gross Unrealized Gains	0.9	1.5
Gross Unrealized Losses	15.9	7.9
Fair Value	2,586.8	2,291.3
Auction Rate
Investment Holdings [Line Items]
Total	2.9	4.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.1
Fair Value	2.9	4.3
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	449.6	439.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	6.3	4.1
Fair Value	443.3	435.1
Other
Investment Holdings [Line Items]
Total	18.1	22.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 18.1	$ 22.3

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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