CONSOLIDATED STATEMENTS OF CASH FLOW (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]
Net income	$ 29,590	$ 40,125	$ 69,633	$ 103,792
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	16,285	15,672	47,289	46,659
Amortization	1,997	1,582	7,017	6,305
Change in deferred taxes and other liabilities	3,074	13,548	12,434	12,802
Provision for write-off of property, plant and equipment	303	(5)	536	1,101
Non-cash interest (income)/expense	(309)	151	(138)	454
Compensation and benefits paid or payable in Class A Common Stock	80	790	596	1,413
Fair value adjustment on foreign currency option	(64)	0	0	0
Provision for credit losses from uncollected receivables and contract assets	(105)	332	1,664	1,136
Foreign currency remeasurement loss/(gain) on intercompany loans	169	(1,049)	15,750	(2,656)
Changes in operating assets and liabilities that provided/(used) cash:
Accounts receivable	(2,048)	(10,282)	6,069	(8,276)
Contract assets	(7,923)	(9,605)	(27,932)	(6,558)
Inventories	4,585	(3,760)	(20,043)	(21,927)
Prepaid expenses and other current assets	(4,532)	131	(6,989)	(4,057)
Income taxes prepaid and receivable	(454)	304	(662)	662
Accounts payable	(5,108)	363	(15,491)	7,837
Accrued liabilities	2,838	3,407	(8,063)	(8,762)
Income taxes payable	1,786	(5,611)	3,741	1,619
Noncurrent receivables	(228)	(339)	169	(679)
Other noncurrent liabilities	111	(2,251)	(413)	(4,411)
Other, net	(388)	(6)	(1,474)	139
Net cash provided by operating activities	39,659	43,497	83,693	126,593
INVESTING ACTIVITIES
Purchases of property, plant and equipment	(9,349)	(13,442)	(31,320)	(48,846)
Purchased software	(109)	(257)	(155)	(306)
Net Cash Provided by (Used in) Investing Activities, Total	(9,458)	(13,699)	(31,475)	(49,152)
FINANCING ACTIVITIES
Proceeds from borrowings	0	0	70,000	20,000
Principal payments on debt	(17,005)	(58,006)	(76,016)	(95,014)
Principal payments on finance lease liabilities	(335)	(298)	(6,798)	(876)
Taxes paid in lieu of share issuance	0	0	(490)	(971)
Proceeds from options exercised	5	33	25	105
Dividends paid	(6,144)	(5,814)	(18,424)	(17,435)
Net cash used in financing activities	(23,479)	(64,085)	(31,703)	(94,191)
Effect of exchange rate changes on cash and cash equivalents	4,545	(7,207)	(751)	(7,266)
Increase/(decrease) in cash and cash equivalents	11,267	(41,494)	19,764	(24,016)
Cash and cash equivalents at beginning of period	204,037	215,233	195,540	197,755
Cash and cash equivalents at end of period	$ 215,304	$ 173,739	$ 215,304	$ 173,739