Derivative Instruments and Fair Value Measurements - Derivatives by hedging relationship and fair value measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives:
Current assets	$ 166	$ 112
Current assets, counterparty offsets	0	(266)
Current liabilities	(145)	(64)
Current liabilities, counterparty offsets	0	266
Net value	21	48
Net value, counterparty offsets	0	0
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1) [Member]
Derivatives:
Current assets	0	0
Current liabilities	0	0
Net value	0	0
Significant Unobservable Inputs (Level 3) [Member]
Derivatives:
Current assets	166	378
Current liabilities	(145)	(330)
Net value	21	48
Significant Unobservable Inputs (Level 3) [Member]
Derivatives:
Current assets	0	0
Current liabilities	0	0
Net value	$ 0	$ 0