Litigation (Details) - USD ($)	Jun. 30, 2015	Mar. 12, 2015	Dec. 31, 2014	Jun. 30, 2014	Aug. 31, 2011	Oct. 31, 2010
Loss Contingencies [Line Items]
Potential liabilities related to litigation and environmental matters	$ 20,700,000		$ 27,600,000	$ 32,100,000
Natural gas gathering operations [Member]
Loss Contingencies [Line Items]
Arbitration award						$ 26,600,000
Litigation related to construction materials [Member]
Loss Contingencies [Line Items]
Loss contingency, estimate of possible loss					$ 3,700,000
Additional amount per day of potential penalties					$ 5,000
Litigation related to construction services [Member]
Loss Contingencies [Line Items]
Damage Repair Estimate		$ 26,000,000