Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Statement [Abstract]
Revenues	$ 178,007	$ 196,500	$ 351,428	$ 390,234
Cost of sales	137,478	161,119	270,894	316,664
Gross profit	40,529	35,381	80,534	73,570
Earnings of unconsolidated mines	13,035	12,076	25,683	24,629
Operating expenses
Selling, general and administrative expenses	47,528	45,219	93,787	91,635
Amortization of intangible assets	826	950	1,808	2,035
Operating expenses	48,354	46,169	95,595	93,670
Operating profit	5,210	1,288	10,622	4,529
Other (income) expense
Interest expense	1,470	1,661	2,975	3,786
Income from other unconsolidated affiliates	(303)	(300)	(606)	(1,472)
Closed mine obligations	349	425	725	827
Other, net, including interest income	2,017	(167)	2,070	312
Other (income) expense	3,533	1,619	5,164	3,453
Income (loss) before income tax provision (benefit)	1,677	(331)	5,458	1,076
Income tax provision (benefit)	(1,439)	(56)	(460)	324
Net income (loss)	$ 3,116	$ (275)	$ 5,918	$ 752
Earnings Per Share [Abstract]
Basic earnings per share (in dollars per share)	$ 0.45	$ (0.04)	$ 0.86	$ 0.11
Diluted earnings per share (in dollars per share)	0.45	(0.04)	0.86	0.11
Dividends per share (in dollars per share)	$ 0.2675	$ 0.2625	$ 0.53	$ 0.5200
Basic weighted average shares outstanding (in shares)	6,856	7,048	6,853	7,114
Diluted weighted average shares outstanding (in shares)	6,874	7,048	6,878	7,129