Commitments, Contingencies and Litigation (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011	Aug. 31, 2011 Litigation related to construction materials [Member]	Dec. 31, 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	$ 22,500,000	$ 64,100,000
Amount of compensatory damages in LPP's notice of demand									149,700,000
Arbitration award						26,600,000		22,000,000
Loss contingency, estimate of possible loss			3,700,000
Additional amount per day of potential penalties			5,000
Amount of natural gas gathering contract dispute charged to operations and maintenance, before tax					(24,100,000)		26,600,000
Amount of natural gas gathering contract dispute charged to operation and maintenance expense, after tax					(15,000,000)		16,500,000
Estimated damages in pending litigation, low estimate				16,100,000
Estimated damages in pending litigation, high estimate				22,600,000
Estimated damages in pending litigation, revised estimate				$ 4,800,000