Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net income	$ 31,440	$ 32,449	$ 59,050	$ 40,043
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	15,971	15,498	32,560	31,004
Amortization	2,280	2,456	4,573	5,020
Change in deferred taxes and other liabilities	974	3,543	5,416	9,360
Impairment of property, plant and equipment	353	36	538	233
Non-cash interest expense	265	20	310	171
Compensation and benefits paid or payable in Class A Common Stock	1,639	1,198	1,626	516
Provision for credit losses from uncollected receivables and contract assets	27	114	(83)	1,769
Foreign currency remeasurement (gain)/loss on intercompany loans	(723)	194	(1,031)	15,581
Fair value adjustment on foreign currency options	1	0	140	64
Changes in operating assets and liabilities that provided/(used) cash:
Accounts receivable	(129)	11,511	(3,365)	8,117
Contract assets	9,539	(11,169)	25,643	(20,009)
Inventories	(1,821)	(4,878)	(10,384)	(24,628)
Prepaid expenses and other current assets	(606)	(301)	(1,505)	(2,457)
Income taxes prepaid and receivable	1,156	29	(309)	(208)
Accounts payable	(4,580)	(9,337)	4,608	(10,383)
Accrued liabilities	2,062	4,171	(17,423)	(10,901)
Income taxes payable	4,121	5,526	(3,956)	1,955
Noncurrent receivables	1,099	628	1,587	397
Other noncurrent liabilities	(2,166)	(464)	(4,263)	(524)
Other, net	1,051	(552)	1,908	(1,086)
Net cash provided by operating activities	61,953	50,672	95,640	44,034
INVESTING ACTIVITIES
Purchases of property, plant and equipment	(10,302)	(9,212)	(22,836)	(21,971)
Purchased software	(286)	0	(288)	(46)
Net cash used in investing activities	(10,588)	(9,212)	(23,124)	(22,017)
FINANCING ACTIVITIES
Proceeds from borrowings	0	0	8,000	70,000
Principal payments on debt	(34,002)	(56,005)	(56,009)	(59,011)
Principal payments on finance lease liabilities	(355)	(329)	(704)	(6,463)
Taxes paid in lieu of share issuance	0	0	(998)	(490)
Proceeds from options exercised	21	20	149	20
Dividends paid	(6,474)	(6,141)	(12,942)	(12,280)
Net cash used in financing activities	(40,810)	(62,455)	(62,504)	(8,224)
Effect of exchange rate changes on cash and cash equivalents	4,904	2,352	2,002	(5,296)
Increase/(decrease) in cash and cash equivalents	15,459	(18,643)	12,014	8,497
Cash and cash equivalents at beginning of period	237,871	222,680	241,316	195,540
Cash and cash equivalents at end of period	$ 253,330	$ 204,037	$ 253,330	$ 204,037