Properties, Plant and Equipment - Oil and Gas Properties - Additional Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) project	Jun. 30, 2016 USD ($) project
Oil and Gas Properties
Number of exploratory wells for which costs were capitalized | project			0	0
Costs capitalized in connection with exploratory wells	$ 0	$ 0	$ 0	$ 0
Exploration expense on exploratory wells			0	0
Amount of capitalized interest on expenditures			200	0
Depletion of oil and gas properties	$ 45,100	$ 37,800	80,500	79,300
Impairments of proved or unproved oil and gas properties			0	$ 0
Impairment charges			$ 161,900