Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Investment Holdings [Line Items]
Amortized Cost	$ 35,018.7	$ 35,631.9
Gross unrealized, gains	92.9	78.7
Gross unrealized, losses	114.0	130.8
Fair Value	34,997.6	35,579.8
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	6,815.5	7,514.5
Gross unrealized, gains	27.9	22.4
Gross unrealized, losses	13.2	14.3
Fair Value	6,830.2	7,522.6
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	953.2	890.8
Gross unrealized, gains	0.2	0.0
Gross unrealized, losses	1.9	5.6
Fair Value	951.5	885.2
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	17,947.4	17,914.1
Gross unrealized, gains	52.6	49.3
Gross unrealized, losses	63.6	70.6
Fair Value	17,936.4	17,892.8
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	219.8	420.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	1.7	2.1
Fair Value	218.1	417.9
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,578.0	3,787.4
Gross unrealized, gains	3.6	2.6
Gross unrealized, losses	22.6	24.8
Fair Value	3,559.0	3,765.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	887.3	1,148.6
Gross unrealized, gains	1.1	0.8
Gross unrealized, losses	3.5	5.5
Fair Value	884.9	1,143.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,761.0	1,343.6
Gross unrealized, gains	3.1	0.9
Gross unrealized, losses	2.8	3.8
Fair Value	1,761.3	1,340.7
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,349.0	2,083.7
Gross unrealized, gains	4.4	2.7
Gross unrealized, losses	1.5	1.3
Fair Value	2,351.9	2,085.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	4.6	5.0
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.3	0.3
Fair Value	4.3	4.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	459.1	474.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	2.9	2.5
Fair Value	456.2	471.6
Other
Investment Holdings [Line Items]
Amortized Cost	43.8	50.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.0	0.0
Fair Value	$ 43.8	$ 50.1

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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