Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net (loss) income	$ 9,986	$ 9,444	$ 16,132	$ 22,115	$ 23,312	$ 33,392	$ 15,778	$ 41,489	$ (2,269)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	3,681	3,776	6,984	7,674	10,865	11,676	15,514	15,270	15,196
Provision for doubtful accounts	2,350	2,637	3,396	3,829	5,000	6,448	6,861	7,607	5,622
Noncash amortization of debt financing costs	342	350	685	701	1,030	1,054	1,393	1,404	1,251
Shared-based compensation expense	761	673	1,479	1,517	2,200	2,297	2,843	3,137	2,503
Deferred income taxes	2,298	2,181	2,263	6,396	1,840	8,369	1,562	5,031	7,709
Noncash loss (gain) on forward exchange contracts	737	(2,489)	1,823	(2,161)	2,092	(2,842)	1,972	(1,468)	(726)
Change in other operating items:
Accounts receivable	(26,356)	(34,884)	(26,552)	(47,306)	(25,454)	(50,389)	11,954	(34,987)	(13,792)
Inventories	(4,739)	5,261	(462)	7,010	1,191	4,507	9,495	4,836	(25,104)
Prepaid expenses	(2,272)	(1,496)	(2,501)	(2,507)	(2,607)	(2,126)	(1,793)	(2,292)	179
Accounts payable	9,548	(2,105)	6,563	2,845	3,272	6,653	(24,261)	1,451	23,250
Accrued liabilities							(3,525)	2,631	(12,284)
Other operating activities, net	(2,307)	(3,363)	(1,061)	788	5,767	1,000	(1,047)	(3,117)	722
Net cash provided by operating activities	(5,971)	(20,015)	8,749	901	28,508	20,039	36,746	40,992	2,257
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property, plant and equipment	(5,580)	(1,716)	(12,800)	(5,158)	(18,743)	(9,823)	(24,002)	(14,150)	(13,458)
Proceeds from disposal/sale of property, plant and equipment	20	0	20	0	20	18	23	49	2,682
Payments for acquisition of business	0		0		(34,000)		(34,000)	0	0
Net cash used in investing activities	(5,560)	(1,716)	(12,780)	(5,158)	(52,723)	(9,805)	(57,979)	(14,101)	(10,776)
CASH FLOWS FROM FINANCING ACTIVITIES:
Borrowings of Revolving Credit Facility	0	36,500	0	80,500	8,500	80,500	35,700	80,500	0
Repayment of Revolving Credit Facility	0	(29,000)	0	(80,500)	(8,500)	(80,500)	(35,700)	(80,500)	0
Borrowings of Term Loan Facility							0	0	175,000
Repayment of Term Loan Facility	(5,244)	(1,094)	(6,338)	(2,188)	(6,338)	(3,281)	(8,525)	(4,375)	(2,188)
Surrender of common stock by employees							(985)	(1,131)	(1,361)
Redemption of Notes							0	0	(235,000)
Prepayment charge for redemption of Notes							0	0	(1,543)
Payment of Term Loan Facility discount							0	0	(3,500)
Payment of debt issuance costs							(160)	0	(4,256)
Other financing activities, net	105		222		381		443	329	0
Net cash used in financing activities	(5,349)	6,406	(6,560)	(2,188)	(6,719)	(3,281)	(10,113)	(5,835)	(72,848)
EFFECT OF CURRENCY EXCHANGE RATE CHANGES ON CASH	315	989	199	(1,125)	(1,276)	(1,672)	(56)	(2,387)	3,451
NET INCREASE (DECREASE) IN CASH	(16,565)	(14,336)	(10,392)	(7,570)	(32,210)	5,281	(31,402)	18,669	(77,916)
CASH:
Beginning of period	70,913	52,244	70,913	52,244	70,913	52,244	70,913	52,244	130,160
End of period	54,348	37,908	60,521	44,674	38,703	57,525	39,511	70,913	52,244
SUPPLEMENTAL CASH FLOW INFORMATION:
Cash paid for interest	3,373	3,408	6,787	6,937	10,212	10,421	13,873	14,046	18,572
Cash paid for income taxes, net	2,593	808	4,180	1,693	5,530	2,081	8,774	3,143	3,276
Unpaid purchases of property and equipment included in accounts payable	$ 233	$ 49	$ 526	$ 416	$ 155	$ 132	$ 624	$ 509	$ 109