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Litigation (Details) (USD $)
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Sep. 30, 2012
Litigation related to construction materials [Member]
Sep. 30, 2012
Natural gas gathering operations [Member]
Jun. 30, 2012
Natural gas gathering operations [Member]
Oct. 31, 2010
Natural gas gathering operations [Member]
Sep. 30, 2010
Natural gas gathering operations [Member]
Jan. 13, 2012
Guarantee obligation under construction contract [Member]
Dec. 31, 2009
Guarantee obligation under construction contract [Member]
Loss Contingencies [Line Items]                    
Potential liabilities related to litigation and environmental matters $ 41,600,000 $ 64,100,000 $ 40,600,000              
Amount of compensatory damages in LPP's notice of demand                   149,700,000
Arbitration award             26,600,000   22,000,000  
Estimated damages in pending litigation, low estimate         16,100,000          
Estimated damages in pending litigation, high estimate         22,600,000          
Loss contingency, estimate of possible loss       3,700,000            
Additional amount per day of potential penalties       5,000            
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, before tax           (24,100,000)   26,600,000    
Amount of expense (benefit) recorded in operation and maintenance expense related to natural gas gathering contract dispute, after tax           $ (15,000,000)   $ 16,500,000