Contingencies (Details) (USD $)													0 Months Ended		9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Sep. 30, 2011 Pending or Threatened Litigation [Member] Natural gas gathering operations [Member]	Apr. 20, 2011 Pending or Threatened Litigation [Member] Natural gas gathering operations [Member]	Oct. 31, 2010 Pending or Threatened Litigation [Member] Natural gas gathering operations [Member]	Sep. 30, 2010 Pending or Threatened Litigation [Member] Natural gas gathering operations [Member]	Sep. 19, 2011 Pending or Threatened Litigation [Member] Guarantee obligation under construction contract [Member]	Dec. 31, 2009 Pending or Threatened Litigation [Member] Guarantee obligation under construction contract [Member]	Jul. 31, 2007 Pending or Threatened Litigation [Member] Guarantee obligation under construction contract [Member]	Sep. 30, 2011 Pending or Threatened Litigation [Member] Litigation related to construction materials [Member]	Jan. 18, 2011 Pending or Threatened Litigation [Member] Litigation related to construction materials [Member]	Aug. 03, 2011 Pending or Threatened Litigation [Member] Natural gas distribution [Member]	Sep. 30, 2011 Portland Harbor Site [Member]	Sep. 30, 2011 Environmental Litigation Number 1 [Member]	Sep. 30, 2011 Environmental Litigation Number 2 [Member]	Sep. 30, 2011 Environmental Litigation Number 3 [Member]	Sep. 30, 2011 Performance Guarantee [Member]	Sep. 30, 2011 Financial Standby Letter of Credit [Member]
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 40,600,000	$ 45,300,000				$ 26,600,000
Bicent's bank letter of credit guaranteeing CEM's obligation										10,000,000
Amount of compensatory damages in LPP's notice of demand									149,700,000
Estimated damages in pending litigation, low estimate				18,800,000								6,000,000
Estimated damages in pending litigation, high estimate				22,600,000								11,000,000
Amount Of Damages Dismissed In Pending Litigation											5,000,000
Estimated Damages Including Interest In Amended Complaint											21,900,000
Amount of natural gas gathering contract dispute charged to operations and maintenance, before tax							26,600,000
Amount of natural gas gathering contract dispute charged to operation and maintenance expense, after tax			16,500,000				16,500,000
Legal Fees And Expenses Awarded In Litigation					293,000
Cash payment to be made pursuant to settlement agreement													425,000
Suspended Penalties Required If Noncompliant (up to)													1,800,000
Amount of counterclaim related to costs of defending LPP arbitration demand								14,000,000
Site Contingency [Line Items]
Estimated investigative costs (in excess of)														70,000,000
Site Contingency, Loss Exposure Not Accrued, Low Estimate															500,000
Site Contingency, Loss Exposure Not Accrued, High Estimate															11,000,000
Site Contingency, Loss Exposure, Low Estimate																340,000
Site Contingency, Loss Exposure, High Estimate																6,400,000
Reserve for environmental remediation of manufactured gas plant site																6,400,000
Site Contingency, Loss Exposure Not Accrued, Best Estimate																	8,000,000
Estimated proportional share of clean-up liability (in hundredths)															50.00%
Guarantor Obligations [Line Items]
Guarantor Obligations, Maximum Exposure, Undiscounted																		86,000,000
2011																		1,600,000
2012																		73,800,000
2013																		1,300,000
2014																		1,400,000
2015																		100,000
2016																		100,000
2018																		800,000
2019																		300,000
Amount Outstanding Under Guarantees That Is Reflected On Balance Sheet																		1,000,000
Expires on a specified number of days after the receipt of written notice																		2,600,000
No scheduled maturity date																		4,000,000
Outstanding letters of credit																			27,400,000
Letters of credit set to expire in 2011																			19,700,000
Letters of credit set to expire in 2012																			7,700,000
Natural gas transportation and storage agreement fixed maximum amount of performance guarantee of a related party																		5,000,000
The amount outstanding by a related party under the guarantee																		1,300,000
Amount of surety bonds outstanding																		$ 569,000,000