Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 35,355	$ 41,352	$ 72,999	$ 81,449
Cost of sales	31,515	32,684	64,078	59,396
Gross profit	3,840	8,668	8,921	22,053
Earnings of unconsolidated operations	13,778	14,143	29,781	30,413
Operating expenses
Selling, general and administrative expenses	12,591	12,788	25,318	25,441
Amortization of intangible assets	792	881	1,569	1,528
Gain on sale of assets	(247)	(19)	(247)	(37)
Operating expenses	13,136	13,650	26,640	26,932
Operating profit	4,482	9,161	12,062	25,534
Other (income) expense
Interest expense	330	222	733	453
Interest income	(129)	(581)	(530)	(1,134)
Income from other unconsolidated affiliates	(79)	(323)	(212)	(645)
Closed mine obligations	390	330	824	696
Gain on equity securities	(1,512)	(261)	(316)	(959)
Other, net	(102)	11	(117)	22
Other (income) expense	(1,102)	(602)	382	(1,567)
Income before income tax (benefit) provision	5,584	9,763	11,680	27,101
Income tax (benefit) provision	(466)	1,788	(536)	4,108
Net income	$ 6,050	$ 7,975	$ 12,216	$ 22,993
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.86	$ 1.14	$ 1.74	$ 3.30
Diluted earnings per share (in dollars per share)	$ 0.86	$ 1.14	$ 1.74	$ 3.29
Basic weighted average shares outstanding (in shares)	7,024	6,986	7,010	6,965
Diluted weighted average shares outstanding (in shares)	7,024	6,986	7,034	6,993