Fair Value Measurement - Financial Instruments Carried at Fair Value by Consolidated Balance Sheet Caption and by Valuation Hierarchy (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Valuation Hierarchy
Current assets	$ 2,648	$ 3,474
Long-term assets	1,261	172
Current liabilities	35,726	36,709
Long-term liabilities	10,117	8,788
Recurring | Commodity derivatives
Valuation Hierarchy
Current assets	2,648	3,474
Long-term assets	1,261	172
Current liabilities	35,726	36,709
Long-term liabilities	10,117	8,788
Recurring | Level 2 | Commodity derivatives
Valuation Hierarchy
Current assets	2,893	3,474
Long-term assets	1,056	56
Current liabilities	29,595	28,946
Long-term liabilities	8,529	7,860
Recurring | Level 3 | Commodity derivatives
Valuation Hierarchy
Current assets	(245)
Long-term assets	205	116
Current liabilities	6,131	7,763
Long-term liabilities	$ 1,588	$ 928