Derivative Instruments - Gains (losses) on derivatives (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in AOCI on derivatives (effective portion), net of tax	$ (127)	$ 1,834	$ 2,718
Gain (loss) reclassified from AOCI into income (effective portion), net of tax	(255)	(416)	(1,900)
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in AOCI on derivatives (effective portion), net of tax	328	1,302	0
Gain (loss) reclassified from AOCI into income (effective portion), net of tax	(87)	(110)	0
Commodity Derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in AOCI on derivatives (effective portion), net of tax	(455)	532	2,718
Gain (loss) reclassified from AOCI into income (effective portion), net of tax	$ (168)	$ (306)	$ (1,900)