CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - Parenthetical - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 793	$ 0
Property, plant and equipment, accumulated amortization, depreciation and amortization	351,561	314,276
Owned and lease mineral rights, accumulated depletion and amortization	34,961	27,877
Other acquired intangibles, accumulated amortization	$ 35,717	$ 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