Asset Retirement Obligations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance as of December 31, 2014			$ 94,060
Liabilities incurred			1,002
Liabilities settled			(712)
Accretion expense	$ 1,700	$ 1,924	5,379	$ 5,650
Reclassification associated with held for sale properties			1,052
Balance as of September 30, 2015	$ 100,781		$ 100,781