CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Operating Activities:
Net income	$ 26,136	$ 27,032
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	11,109	10,547
Equity compensation expense	2,673	1,020
Deferred income taxes	1,342	3,516
Equity in undistributed income of unconsolidated joint ventures	(1,007)	(1,009)
Dividends received from unconsolidated joint ventures	1,809	616
Pension and postretirement benefits	(398)	(365)
Loss (gain) from the sale of property, plant and equipment	0	(947)
Bad debt expense	(386)	384
Proceeds from insurance related to operations	0	480
Changes in operating assets and liabilities, excluding effects of acquisitions:
Accounts receivable	(7,822)	(12,439)
Contract assets	(22,040)	0
Inventories	5,813	(769)
Pension and postretirement benefit contributions	(328)	(70)
Other current assets	(1,438)	(4,063)
Accounts payable and other accrued expenses	(13,050)	(1,441)
Other, net	6,355	742
Net cash provided by operating activities	8,768	23,234
Investing Activities:
Acquisition of business, net of cash received	0	(60,191)
Capital expenditures	(9,122)	(5,270)
Proceeds from insurance claims	0	450
Proceeds from the sale of property, plant and equipment, net	0	1,603
Net cash used in investing activities	(9,122)	(63,408)
Financing Activities:
Line of credit issuance costs	0	(1,029)
Repayment of long-term lease obligation	(149)	(87)
Repurchases of capital stock	(2,999)	0
Proceeds from sale of exercise of stock options, net	473	765
Payments of taxes related to net share settlement of equity awards	(6,407)	(2,572)
Proceeds from issuance of shares to employee stock purchase plan	541	422
Net cash used in financing activities	(8,541)	(2,501)
Effect of exchange rate fluctuations on cash	755	1,019
Net decrease in cash and cash equivalents	(8,140)	(41,656)
Cash and cash equivalents at beginning of year	181,159	227,767	$ 227,767
Cash and cash equivalents at end of year	173,019	186,111	181,159
Supplemental Disclosures:
Interest, net of amounts capitalized	$ 1,329	1,209
Income taxes		$ 4,756	$ 5,645