Accumulated Other Comprehensive Loss (Narrative) (Details) - Millennium Pipeline Company, L.L.C [Member] - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Loss [Line Items]
Other Comprehensive Income Unrecognized Gain Loss On Derivatives Arising During Period Net Of Tax	$ 13.9	$ 16.6
Debt Instrument, Maturity Date	Jun. 01, 2025

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration