Commitments And Contingencies - Other Commitments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies
Asset retirement obligations	$ 69,757	$ 54,907	$ 54,727
Acquisition-related contingent consideration	42,614
Material purchase commitment	$ 2,500
Minimum service requirements [Member]
Commitments and Contingencies
Purchase commitment remaining period	3 years
Material purchase commitment	$ 800