Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(2))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 9

-Section 03

-Paragraph 13

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Number 210

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-Subparagraph a(3)

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a)(3))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

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- Definition

The effective interest rate at the end of the reporting period.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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

-Publisher FASB

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-Subparagraph (SX 210.5-02.19(b),22(b))

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Debt (Details 2) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012 Commercial Paper	Dec. 31, 2011 Commercial Paper	Jun. 30, 2012 Senior Unsecured Credit Facility	Dec. 31, 2011 Credit Facility	Nov. 30, 2011 Credit Facility
Debt
Amount outstanding	$ 390.4	$ 401.4
Effective interest rate (as a percent)	0.50%
Maximum borrowing capacity			1,000.0		125.0
Outstanding uncommitted facility				$ 125.0