Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues	$ 45,896	$ 35,355	$ 91,001	$ 72,999
Cost of sales	36,911	31,515	74,324	64,078
Gross profit	8,985	3,840	16,677	8,921
Earnings of unconsolidated operations	13,542	13,778	28,884	29,781
Operating expenses
Selling, general and administrative expenses	12,878	12,591	26,641	25,318
Amortization of intangible assets	911	792	1,893	1,569
Loss (gain) on sale of assets	68	(247)	27	(247)
Operating expenses	13,857	13,136	28,561	26,640
Operating profit	8,670	4,482	17,000	12,062
Other (income) expense
Interest expense	359	330	715	733
Interest income	(100)	(129)	(220)	(530)
Closed mine obligations	364	390	747	824
Gain on equity securities	(1,262)	(1,512)	(2,085)	(316)
Other, net	(127)	(181)	(257)	(329)
Other (income) expense	(766)	(1,102)	(1,100)	382
Income before income tax provision (benefit)	9,436	5,584	18,100	11,680
Income tax provision (benefit)	2,931	(466)	2,634	(536)
Net income	$ 6,505	$ 6,050	$ 15,466	$ 12,216
Earnings per share:
Basic earnings per share (in dollars per share)	$ 0.91	$ 0.86	$ 2.17	$ 1.74
Diluted earnings per share (in dollars per share)	$ 0.91	$ 0.86	$ 2.16	$ 1.74
Basic weighted average shares outstanding (in shares)	7,153	7,024	7,123	7,010
Diluted weighted average shares outstanding (in shares)	7,153	7,024	7,147	7,034