Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Amortized Cost	$ 35,871.8	$ 32,367.8
Gross unrealized, gains	143.9	50.3
Gross unrealized, losses	42.1	100.2
Fair Value	35,973.6	32,317.9
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	7,610.1	6,180.4
Gross unrealized, gains	54.7	3.4
Gross unrealized, losses	0.9	5.5
Fair Value	7,663.9	6,178.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	776.4	36.4
Gross unrealized, gains	0.5	0.1
Gross unrealized, losses	2.1	0.1
Fair Value	774.8	36.4
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	17,978.4	16,370.5
Gross unrealized, gains	68.9	42.8
Gross unrealized, losses	22.9	46.5
Fair Value	18,024.4	16,366.8
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	419.9	309.5
Gross unrealized, gains	0.2	0.1
Gross unrealized, losses	0.0	0.1
Fair Value	420.1	309.5
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,912.6	3,744.4
Gross unrealized, gains	8.7	0.9
Gross unrealized, losses	12.9	33.1
Fair Value	3,908.4	3,712.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,243.4	1,873.3
Gross unrealized, gains	2.1	1.8
Gross unrealized, losses	1.0	4.9
Fair Value	1,244.5	1,870.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,169.6	860.9
Gross unrealized, gains	2.1	0.5
Gross unrealized, losses	1.7	2.0
Fair Value	1,170.0	859.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,248.9	2,504.2
Gross unrealized, gains	5.4	0.1
Gross unrealized, losses	0.2	4.2
Fair Value	2,254.1	2,500.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	6.1	16.7
Gross unrealized, gains	0.0	0.5
Gross unrealized, losses	0.3	0.1
Fair Value	5.8	17.1
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	447.5	378.1
Gross unrealized, gains	1.3	0.0
Gross unrealized, losses	0.1	3.7
Fair Value	448.7	374.4
Other
Investment Holdings [Line Items]
Amortized Cost	58.9	93.4
Gross unrealized, gains	0.0	0.1
Gross unrealized, losses	0.0	0.0
Fair Value	$ 58.9	$ 93.5

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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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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