Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Revenues	$ 181,076	$ 178,007	$ 349,658	$ 351,428
Cost of sales	137,648	137,478	267,095	270,894
Gross profit	43,428	40,529	82,563	80,534
Earnings of unconsolidated mines	13,475	13,035	28,430	25,683
Operating expenses
Selling, general and administrative expenses	47,999	46,870	94,192	92,885
(Gain)/loss on sale of assets	(2,641)	658	(3,044)	902
Amortization of intangible assets	964	826	1,896	1,808
Operating expenses	46,322	48,354	93,044	95,595
Operating profit	10,581	5,210	17,949	10,622
Other expense (income)
Interest expense	1,390	1,470	2,737	2,975
Income from other unconsolidated affiliates	(311)	(303)	(619)	(606)
Closed mine obligations	352	349	735	725
Other, net, including interest income	(326)	2,017	(1,028)	2,070
Other (income) expense	1,105	3,533	1,825	5,164
Income before income tax provision (benefit)	9,476	1,677	16,124	5,458
Income tax provision (benefit)	2,688	(1,439)	4,358	(460)
Net income	$ 6,788	$ 3,116	$ 11,766	$ 5,918
Earnings Per Share [Abstract]
Basic earnings per share (in dollars per share)	$ 0.99	$ 0.45	$ 1.73	$ 0.86
Diluted earnings per share (in dollars per share)	0.99	0.45	1.72	0.86
Dividends per share (in dollars per share)	$ 0.2725	$ 0.2675	$ 0.54	$ 0.5300
Basic weighted average shares outstanding (in shares)	6,835	6,856	6,818	6,853
Diluted weighted average shares outstanding (in shares)	6,850	6,874	6,847	6,878