Revenue Recognition - Summary of Impact of Adoption of Accounting Standards (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2017		Sep. 30, 2018
Revenues
Net commodity sales	$ 65,519	$ 75,947		$ 157,803		$ 181,835
Revenue from Contracts with Customers (“ASC 606”) | Balances without Adoption of ASC 606
Revenues
Net commodity sales	70,112					193,751
Revenue from Contracts with Customers (“ASC 606”) | Effect of Change
Revenues
Net commodity sales	(4,593)					(11,916)
Transportation and processing
Costs and expenses
Transportation and processing	4,593	$ 2,942	[1]	$ 6,370	[1]	11,916
Transportation and processing | Revenue from Contracts with Customers (“ASC 606”) | Balances without Adoption of ASC 606
Costs and expenses
Transportation and processing	(4,593)					(11,916)
Transportation and processing | Revenue from Contracts with Customers (“ASC 606”) | Effect of Change
Costs and expenses
Transportation and processing	$ 4,593					$ 11,916

[1] See “Note 5—Revenue recognition.”