Property and Equipment - Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
ARO liability beginning balance	$ 2,329	$ 2,469	$ 2,464
Liabilities incurred	18	162	39
Accretion of discount	58	92	93
Liabilities settled	(261)	(394)	(127)
Deconsolidation of subsidiary	(871)	0	0
ARO liability ending balance	$ 1,273	$ 2,329	$ 2,469