Asset Retirement Obligations (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Changes in the Company's asset retirement obligations
Beginning asset retirement obligations				$ 11,951
Liabilities incurred during period				2,357
Liabilities settled during period				(252)
Revisions in estimated cash flows				(920)
Accretion expense		$ 132	$ 123	244	$ 241
Ending asset retirement obligations		13,380		13,380
Less: current asset retirement obligations	[1]	(275)		(275)
Long-term asset retirement obligations		$ 13,105		$ 13,105		$ 11,640

[1] Included in accrued liabilities in the Company’s unaudited condensed consolidated balance sheet at June 30, 2015.