Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 32,603	$ 31,440	$ 114,052	$ 96,321
Cost of sales	26,416	25,345	85,812	79,956
Gross profit	6,187	6,095	28,240	16,365
Earnings of unconsolidated operations	17,438	17,141	47,851	48,119
Operating expenses
Selling, general and administrative expenses	14,341	12,032	39,782	34,522
Amortization of intangible assets	715	714	2,243	2,212
Gain on sale of assets	(94)	(57)	(131)	(320)
Operating expenses	14,962	12,689	41,894	36,414
Operating profit	8,663	10,547	34,197	28,070
Other (income) expense
Interest expense	230	421	683	1,636
Interest income	(1,878)	(94)	(3,012)	(326)
Income from other unconsolidated affiliates	(327)	(321)	(972)	(954)
Closed mine obligations	383	272	1,079	994
Gain on equity securities	(108)	(397)	(1,067)	(481)
Other, net	(1,258)	8	(1,236)	(18)
Other (income) expense	(2,958)	(111)	(4,525)	851
Income before income tax provision	11,621	10,658	38,722	27,219
Income tax provision	1,357	1,458	5,465	3,450
Net income	$ 10,264	$ 9,200	$ 33,257	$ 23,769
Earnings per share:
Basic earnings per share (in dollars per share)	$ 1.47	$ 1.33	$ 4.77	$ 3.43
Diluted earnings per share (in dollars per share)	$ 1.47	$ 1.33	$ 4.76	$ 3.43
Basic weighted average shares outstanding (in shares)	6,991	6,940	6,973	6,921
Diluted weighted average shares outstanding (in shares)	6,991	6,940	6,992	6,939