Balance Sheet Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Foreign currency translation	$ 1,273	$ 1,178
Derivative instruments designated as cash flow hedges	(9,868)	(9,981)
Reclassification of certain tax effects from AOCL	(2,143)	0
Postretirement medical plans	31	31
Accumulated other comprehensive loss	$ (10,707)	$ (8,772)