CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - Parenthetical - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 432	$ 0
Property, plant and equipment, accumulated amortization, depreciation and amortization	395,397	314,276
Owned and lease mineral rights, accumulated depletion and amortization	40,623	27,877
Other acquired intangibles, accumulated amortization	$ 38,806	$ 32,686
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	5,000,000.0	5,000,000.0
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in dollars per share	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	50,000,000.0	50,000,000.0
Common stock, shares issued (in shares)	20,600,000	20,500,000
Common stock, shares outstanding (in shares)	18,300,000	18,200,000
Treasury stock, shares at cost (in shares)	2,300,000	2,300,000