CONSOLIDATED STATEMENTS OF CASH FLOW (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
OPERATING ACTIVITIES
Net income	$ 34,259	$ 29,843	$ 63,667	$ 37,742
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	15,345	17,114	30,987	35,416
Amortization	2,409	2,559	4,723	5,205
Change in deferred taxes and other liabilities	319	(6,183)	(746)	(8,211)
Provision for write-off of property, plant and equipment	720	853	1,106	1,124
Non-cash interest expense	152	154	303	154
Compensation and benefits paid or payable in Class A Common Stock	1,170	1,047	623	1,336
Fair value adjustment on foreign currency option		34		71
Changes in operating assets and liabilities that provided cash:
Accounts receivable	14,434	(12,052)	2,810	(37,141)
Contract assets	3,528	(13,877)	3,047	(11,761)
Inventories	(1,505)	(1,687)	(18,167)	(13,440)
Prepaid expenses and other current assets	(1,384)	(1,157)	(4,188)	(5,220)
Income taxes prepaid and receivable	(316)	(5)	358	97
Accounts payable	(14,276)	11,420	7,474	8,882
Accrued liabilities	(1,074)	5,846	(12,169)	4,619
Income taxes payable	5,724	10,020	7,230	6,589
Noncurrent receivables	(46)	(1,643)	(340)	(4,170)
Other noncurrent liabilities	(481)	(854)	(2,160)	(1,231)
Other, net	(448)	(5,745)	(1,462)	(3,321)
Net cash provided by operating activities	58,530	35,687	83,096	16,740
INVESTING ACTIVITIES
Purchases of property, plant and equipment	(14,606)	(23,352)	(35,404)	(39,123)
Purchased software	(27)	(23)	(49)	(52)
Net cash used in investing activities	(14,633)	(23,375)	(35,453)	(39,175)
FINANCING ACTIVITIES
Proceeds from borrowings		10,020	20,000	23,031
Principal payments on debt	(9,004)	(5,653)	(37,008)	(14,143)
Principal payments on finance lease liabilities	(178)		(578)
Taxes paid in lieu of share issuance			(971)	(1,652)
Proceeds from options exercised	28	3	72	150
Dividends paid	(5,813)	(5,482)	(11,621)	(10,956)
Net cash used in financing activities	(14,967)	(1,112)	(30,106)	(3,570)
Effect of exchange rate changes on cash and cash equivalents	(1,082)	(7,882)	(59)	(2,978)
Increase/(decrease) in cash and cash equivalents	27,848	3,318	17,478	(28,983)
Cash and cash equivalents at beginning of period	187,385	151,426	197,755	183,727
Cash and cash equivalents at end of period	$ 215,233	$ 154,744	$ 215,233	$ 154,744