Consolidated Statements of Cash Flows - USD ($)	9 Months Ended
	Sep. 25, 2020	Sep. 27, 2019
Cash flows from operating activities:
Net income	$ 10,071,000	$ 35,909,000
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation, depletion, and amortization	32,042,000	31,955,000
Amortization of deferred financing costs in interest expense	547,000	720,000
Stock-based compensation expense (non-cash)	3,989,000	5,230,000
Deferred income tax expense (benefit)	(5,187,000)	5,725,000
Net pension curtailments and settlements	94,000	3,296,000
Impairment charges	10,766,000	14,141,000
Changes in assets and liabilities, net of acquired assets and liabilities:
Decrease (increase) in accounts receivable	13,899,000	(36,146,000)
Decrease (increase) in inventory	(9,883,000)	22,089,000
Decrease (increase) in prepaid and other current assets	(686,000)	(416,000)
Increase (decrease) in accounts payable and accrued expenses	(9,759,000)	(14,114,000)
Increase (decrease) in unearned revenue	(298,000)	(728,000)
Increase (decrease) in interest and taxes payable	143,000	(1,499,000)
Increase (decrease) in unearned income due to customer prepayments	40,385,000	0
Domestic pension plan contributions	0	(4,500,000)
Other-net	(6,679,000)	(2,126,000)
Net cash provided by operating activities	79,444,000	59,536,000
Cash flows from investing activities:
Payments for acquisition, net of cash acquired	(130,715,000)	0
Payments for purchase of property, plant, and equipment	(46,285,000)	(18,193,000)
Payments for mine development	0	1,903,000
Derivative, Cash Received on Hedge	(3,249,000)	0
Proceeds from sale of property, plant, and equipment	35,000	17,000
Net cash used in investing activities	(173,716,000)	(20,079,000)
Cash flows from financing activities:
Short-term debt under revolving credit agreement, net	120,000,000	0
Repayment of long-term debt	(16,357,000)	(599,000)
Principal payments under finance lease obligations	1,440,000	894,000
Cash dividends paid	(6,920,000)	(6,612,000)
Repurchase of common stock	(6,766,000)	(199,000)
Payments of withholding taxes for stock-based compensation awards	(2,212,000)	(4,832,000)
Deferred financing costs	0	(2,018,000)
Net cash provided by (used in) financing activities	86,305,000	(15,154,000)
Effects of exchange rate changes	714,000	(422,000)
Net change in cash and cash equivalents	(7,253,000)	23,881,000
Cash and cash equivalents at beginning of period	125,007,000	70,645,000
Cash and cash equivalents at end of period	$ 117,754,000	$ 94,526,000