CONSOLIDATED STATEMENTS OF CASH FLOW (Unaudited) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net income	$ 32,449	$ 7,594	$ 34,259	$ 29,408	$ 40,043	$ 63,667
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	15,498		15,345		31,004	30,987
Amortization	2,456		2,409		5,020	4,723
Change in deferred taxes and other liabilities	3,543		319		9,360	(746)
Provision for write-off of property, plant and equipment	36		720		233	1,106
Non-cash interest expense	20		152		171	303
Compensation and benefits paid or payable in Class A Common Stock	1,198		1,170		516	623
Fair value adjustment on foreign currency option	0		0		64	0
Provision for credit losses from uncollected receivables and contract assets	114		219		1,769	804
Foreign currency remeasurement loss/(gain) on intercompany loans	194		100		15,581	(1,607)
Increase (Decrease) in Operating Capital [Abstract]
Accounts receivable	11,511		14,215		8,117	2,006
Contract assets	(11,169)		3,528		(20,009)	3,047
Inventories	(4,878)		(1,505)		(24,628)	(18,167)
Prepaid expenses and other current assets	(301)		(1,384)		(2,457)	(4,188)
Income taxes prepaid and receivable	29		(316)		(208)	358
Accounts payable	(9,337)		(14,276)		(10,383)	7,474
Accrued liabilities	4,171		(1,074)		(10,901)	(12,169)
Income taxes payable	5,526		5,724		1,955	7,230
Noncurrent receivables	628		(46)		397	(340)
Other noncurrent liabilities	(464)		(481)		(524)	(2,160)
Other, net	(552)		(548)		(1,086)	145
Net cash provided by operating activities	50,672		58,530		44,034	83,096
Net Cash Provided by (Used in) Investing Activities [Abstract]
Purchases of property, plant and equipment	(9,212)		(14,606)		(21,971)	(35,404)
Purchased software	0		(27)		(46)	(49)
Net Cash Provided by (Used in) Investing Activities, Total	(9,212)		(14,633)		(22,017)	(35,453)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Proceeds from borrowings	0		0		70,000	20,000
Principal payments on debt	(56,005)		(9,004)		(59,011)	(37,008)
Principal payments on finance lease liabilities	(329)		(178)		(6,463)	(578)
Taxes paid in lieu of share issuance	0		0		(490)	(971)
Proceeds from options exercised	20		28		20	72
Dividends paid	(6,141)		(5,813)		(12,280)	(11,621)
Net cash used in financing activities	(62,455)		(14,967)		(8,224)	(30,106)
Effect of exchange rate changes on cash and cash equivalents	2,352		(1,082)		(5,296)	(59)
Increase/(decrease) in cash and cash equivalents	(18,643)		27,848		8,497	17,478
Increase/(decrease) in cash and cash equivalents	222,680	195,540	187,385	197,755	195,540	197,755	$ 197,755
Cash and cash equivalents at beginning of period	$ 204,037	$ 222,680	$ 215,233	$ 187,385	$ 204,037	$ 215,233	$ 195,540