Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Condensed Consolidated Statements of Comprehensive Income (unaudited) [Abstract]
Net earnings	$ 19,214	$ 16,043	$ 58,239	$ 48,096
Other comprehensive income, net of tax:
Net foreign currency translation adjustment	(545)	1,502	(2,093)	4,740
Net change in postretirement benefit plans, net of taxes of $(8) and $(5) for the three months ended September 30, 2018 and 2017, and $(12) and $(14) for the nine months ended September 30, 2018 and 2017.	9	10	38	31
Other comprehensive (loss)/income	(536)	1,512	(2,055)	4,771
Comprehensive income	$ 18,678	$ 17,555	$ 56,184	$ 52,867