Guarantees (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
GuarantorObligationsAmountsIssuedToRelatedParty	$ 62
GuaranteeObligationsAmountThatWillExpire	350
Guarantor Obligations, Current Carrying Value	47	19
Performance Guarantee [Member]
Guarantor Obligations, Maximum Exposure, Undiscounted	283
Indemnification Agreement [Member] \| Spectra Capital [Member]
Guarantor Obligations, Maximum Exposure, Undiscounted	206
Indemnification Agreement [Member] \| Progress Energy [Member]
Guarantor Obligations, Maximum Exposure, Undiscounted	219
Payment Guarantee [Member] \| Progress Energy [Member]
GuarantorObligationsAmountsIssuedToRelatedParty	$ 300

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 9

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name FASB Interpretation (FIN)

-Number 45

-Paragraph 13

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant