Statements of Consolidated Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive Income Net Of Tax [Abstract]
Net income	$ 5,264	$ 9,054	$ 6,735	$ 5,528	$ 702	$ 6,278	$ 4,156	$ 1,518	$ 26,581	$ 12,654	$ 15,255
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustment									(12,285)	10,070	(3,579)
Recognized net actuarial gains									386	269	326
Gain (loss) on unfunded pension obligations									(244)	(410)	35
Other comprehensive income (loss), net of tax									(12,143)	9,929	(3,218)
Comprehensive income									$ 14,438	$ 22,583	$ 12,037