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Commitments, contingencies and litigation (Details) (USD $)
Dec. 31, 2013
Dec. 31, 2012
Aug. 31, 2011
Litigation related to construction materials [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 $ 29,500,000 $ 22,500,000            
Amount of compensatory damages in LPP's notice of demand               149,700,000
Arbitration award         26,600,000   22,000,000  
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    
Loss contingency, estimate of possible loss     3,700,000          
Additional amount per day of potential penalties     $ 5,000