Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenues	$ 239,107	$ 221,714	$ 629,341	$ 599,497
Cost of sales	196,892	175,171	513,556	480,260
Gross profit	42,215	46,543	115,785	119,237
Earnings of unconsolidated mines	12,234	12,064	36,863	36,069
Operating expenses
Selling, general and administrative expenses	47,551	46,373	139,186	145,792
Amortization of intangible assets	1,138	911	3,173	2,667
Operating expenses	48,689	47,284	142,359	148,459
Operating profit	5,760	11,323	10,289	6,847
Other expense (income)
Interest expense	1,597	2,046	5,383	5,450
Income from other unconsolidated affiliates	(264)	(191)	(1,736)	(159)
Closed mine obligations	244	316	1,071	940
Other, net, including interest income	908	86	1,220	(65)
Other (income) expense	2,485	2,257	5,938	6,166
Income before income tax provision (benefit)	3,275	9,066	4,351	681
Income tax provision (benefit)	134	1,367	458	(1,870)
Net income	$ 3,141	$ 7,699	$ 3,893	$ 2,551
Earnings Per Share [Abstract]
Basic earnings per share (in dollars per share)	$ 0.45	$ 1.02	$ 0.55	$ 0.33
Diluted earnings per share (in dollars per share)	0.45	1.02	0.55	0.33
Dividends per share (in dollars per share)	$ 0.2625	$ 0.2575	$ 0.7825	$ 0.7650
Basic weighted average shares outstanding	6,924	7,515	7,051	7,688
Diluted weighted average shares outstanding	6,935	7,533	7,065	7,702