Derivative Financial Instruments and Fair Value Measurements - Derivative Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Commodities contracts - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset — Commodities contracts	$ 2,228	$ 1,431
Liability — Commodities contracts	1,450	1,844
Level 1
Derivatives, Fair Value [Line Items]
Asset — Commodities contracts	0	0
Liability — Commodities contracts	0	0
Level 2
Derivatives, Fair Value [Line Items]
Asset — Commodities contracts	2,228	1,431
Liability — Commodities contracts	1,450	1,844
Level 3
Derivatives, Fair Value [Line Items]
Asset — Commodities contracts	0	0
Liability — Commodities contracts	$ 0	$ 0