Derivative Instruments and Fair Value Measurements - Derivatives by hedging relationship and fair value measurements (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives:
Current assets	$ 162	$ 166
Current assets, counterparty offsets	0	0
Current liabilities	(139)	(145)
Current liabilities, counterparty offsets	0	0
Net value	23	21
Net value, counterparty offsets	0	0
Quoted Prices in Active Markets for Identical Assets and Liabilities (Level 1)
Derivatives:
Current assets	0	0
Current liabilities	0	0
Net value	0	0
Significant Unobservable Inputs (Level 3)
Derivatives:
Current assets	162	166
Current liabilities	(139)	(145)
Net value	23	21
Significant Unobservable Inputs (Level 3)
Derivatives:
Current assets	0	0
Current liabilities	0	0
Net value	$ 0	$ 0