Contingencies (Details) (USD $)								6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Pending or Threatened Litigation [Member] Natural gas gathering operations [Member]	Jun. 30, 2011 Pending or Threatened Litigation [Member] Guarantee obligation under construction contract [Member]	Jun. 30, 2011 Pending or Threatened Litigation [Member] Litigation related to construction materials [Member]	Jun. 30, 2011 Pending or Threatened Litigation [Member] Natural gas distribution [Member]	Jun. 30, 2011 Portland Harbor Site [Member]	Jun. 30, 2011 Environmental Litigation Number 1 [Member]	Jun. 30, 2011 Environmental Litigation Number 2 [Member]	Jun. 30, 2011 Environmental Litigation Number 3 [Member]	Jun. 30, 2011 Performance Guarantee [Member]	Jun. 30, 2011 Financial Standby Letter of Credit [Member]
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 40,700,000	$ 45,300,000	$ 26,600,000
Number Of Mega Watts Of Combined Cycle Electric Generating Facility				550
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
Amount of indemnifiable losses sought under indemnification agreement (in excess of)				10,000,000
Estimated damages in pending litigation regarding pavement work under a subcontract, low estimate					6,000,000
Estimated damages in pending litigation regarding pavement work under a subcontract, high estimate					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 operation and maintenance expense, after tax			16,500,000
Amount of natural gas gathering contract dispute charged to operations and maintenance, before tax			26,600,000
Legal Fees And Expenses Awarded In Litigation			293,000
Number of alleged pipeline safety regulation violations (in excess of)						360
Cash payment to be made pursuant to settlement agreement						425,000
Suspended Penalties Required If Noncompliant (up to)						1,800,000
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
Guarantor Obligations [Line Items]
Number of purchase and sale agreements indemnified by Company for its indirect wholly owned subsidiaries (in number of agreements)											3
Maximum number of years from the date of sale agreement that payment is guaranteed (in years)											10
Amount of hedging obligations outstanding reflected on balance sheet and guaranteed by a related party											13,500,000
Guarantor Obligations, Maximum Exposure, Undiscounted											127,600,000
2011											27,800,000
2012											73,400,000
2013											18,200,000
2014											300,000
2015											100,000
2016											100,000
2018											800,000
2019											300,000
Amount Outstanding Under Guarantees That Is Reflected On Balance Sheet											1,300,000
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												21,600,000
Letters of credit set to expire in 2012												5,800,000
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											1,400,000
Amount of surety bonds outstanding											$ 688,000,000