Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Amortized Cost	$ 33,259.9	$ 32,367.8
Gross unrealized, gains	128.4	50.3
Gross unrealized, losses	58.1	100.2
Fair Value	33,330.2	32,317.9
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	6,803.3	6,180.4
Gross unrealized, gains	53.8	3.4
Gross unrealized, losses	0.0	5.5
Fair Value	6,857.1	6,178.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	190.5	36.4
Gross unrealized, gains	0.3	0.1
Gross unrealized, losses	1.0	0.1
Fair Value	189.8	36.4
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,866.1	16,370.5
Gross unrealized, gains	56.6	42.8
Gross unrealized, losses	30.1	46.5
Fair Value	16,892.6	16,366.8
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	280.5	309.5
Gross unrealized, gains	0.2	0.1
Gross unrealized, losses	0.0	0.1
Fair Value	280.7	309.5
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,865.3	3,744.4
Gross unrealized, gains	10.3	0.9
Gross unrealized, losses	17.8	33.1
Fair Value	3,857.8	3,712.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,377.6	1,873.3
Gross unrealized, gains	2.1	1.8
Gross unrealized, losses	4.9	4.9
Fair Value	1,374.8	1,870.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,069.5	860.9
Gross unrealized, gains	2.6	0.5
Gross unrealized, losses	1.9	2.0
Fair Value	1,070.2	859.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,300.4	2,504.2
Gross unrealized, gains	1.2	0.1
Gross unrealized, losses	1.5	4.2
Fair Value	2,300.1	2,500.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	16.6	16.7
Gross unrealized, gains	0.5	0.5
Gross unrealized, losses	0.5	0.1
Fair Value	16.6	17.1
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	406.5	378.1
Gross unrealized, gains	0.7	0.0
Gross unrealized, losses	0.3	3.7
Fair Value	406.9	374.4
Other
Investment Holdings [Line Items]
Amortized Cost	83.6	93.4
Gross unrealized, gains	0.1	0.1
Gross unrealized, losses	0.1	0.0
Fair Value	$ 83.6	$ 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
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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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