Litigation (Details) (USD $)	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Aug. 31, 2011 Litigation related to construction materials [Member]	Jun. 30, 2013 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	$ 32,900,000	$ 22,500,000	$ 48,100,000
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
Revised damage calculation					$ 1,000,000