Property and Equipment - Fair Value of Impairment Provisions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Oil and natural gas property impairments	$ 3	$ 313	$ 12,082
Oil and Gas (Successful Efforts) [Member]
Property, Plant and Equipment [Line Items]
Oil and natural gas property impairments	0	30	10,324
Non Producing Oil and Gas Properties [Member]
Property, Plant and Equipment [Line Items]
Oil and natural gas property impairments	$ 3	$ 283	$ 1,758