Derivative Instruments - Gains (losses) on derivatives (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(loss) recognized in Other comprehensive income on derivatives (effective portion), net of tax	$ 152	$ (7)	$ 1,664	$ (622)
Gain/(loss) reclassified from Accumulated other comprehensive loss into income (effective portion), net of tax	160	(134)	175	(171)
Interest rate swap
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(loss) recognized in Other comprehensive income on derivatives (effective portion), net of tax	145	37	1,413	(318)
Gain/(loss) reclassified from Accumulated other comprehensive loss into income (effective portion), net of tax	169	(7)	379	(133)
Commodity derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain/(loss) recognized in Other comprehensive income on derivatives (effective portion), net of tax	7	(44)	251	(304)
Gain/(loss) reclassified from Accumulated other comprehensive loss into income (effective portion), net of tax	$ (9)	$ (127)	$ (204)	$ (38)