Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Investment Holdings [Line Items]
Amortized Cost	$ 34,507.9	$ 32,367.8
Gross unrealized, gains	165.5	50.3
Gross unrealized, losses	41.3	100.2
Fair Value	34,632.1	32,317.9
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	6,818.3	6,180.4
Gross unrealized, gains	71.0	3.4
Gross unrealized, losses	0.0	5.5
Fair Value	6,889.3	6,178.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	641.7	36.4
Gross unrealized, gains	1.9	0.1
Gross unrealized, losses	0.0	0.1
Fair Value	643.6	36.4
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	17,396.1	16,370.5
Gross unrealized, gains	65.1	42.8
Gross unrealized, losses	24.2	46.5
Fair Value	17,437.0	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	3,997.2	3,744.4
Gross unrealized, gains	15.3	0.9
Gross unrealized, losses	11.1	33.1
Fair Value	4,001.4	3,712.2
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,377.8	1,873.3
Gross unrealized, gains	2.2	1.8
Gross unrealized, losses	3.4	4.9
Fair Value	1,376.6	1,870.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,119.6	860.9
Gross unrealized, gains	3.4	0.5
Gross unrealized, losses	1.5	2.0
Fair Value	1,121.5	859.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,388.0	2,504.2
Gross unrealized, gains	3.5	0.1
Gross unrealized, losses	0.9	4.2
Fair Value	2,390.6	2,500.1
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	6.3	16.7
Gross unrealized, gains	0.0	0.5
Gross unrealized, losses	0.2	0.1
Fair Value	6.1	17.1
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	408.0	378.1
Gross unrealized, gains	2.9	0.0
Gross unrealized, losses	0.0	3.7
Fair Value	410.9	374.4
Other
Investment Holdings [Line Items]
Amortized Cost	74.4	93.4
Gross unrealized, gains	0.0	0.1
Gross unrealized, losses	0.0	0.0
Fair Value	$ 74.4	$ 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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