Unaudited Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Revenues	$ 220,792	$ 239,107	$ 572,220	$ 629,341
Cost of sales	169,084	196,892	439,978	513,556
Gross profit	51,708	42,215	132,242	115,785
Earnings of unconsolidated mines	15,102	12,234	40,785	36,863
Operating expenses
Selling, general and administrative expenses	47,504	47,551	141,291	139,186
Centennial asset impairment charge		0	17,443	0
Amortization of intangible assets	1,164	1,138	2,972	3,173
Operating expenses	66,111	48,689	161,706	142,359
Operating profit	699	5,760	11,321	10,289
Other (income) expense
Interest expense	1,322	1,597	4,297	5,383
Income from other unconsolidated affiliates	(307)	(264)	(913)	(1,736)
Closed mine obligations	223	244	948	1,071
Other, net, including interest income	447	908	2,517	1,220
Other (income) expense	1,685	2,485	6,849	5,938
Income (loss) before income tax provision (benefit)	(986)	3,275	4,472	4,351
Income tax provision (benefit)	(544)	134	(1,004)	458
Net income (loss)	$ (442)	$ 3,141	$ 5,476	$ 3,893
Earnings Per Share [Abstract]
Basic earnings (loss) per share (in dollars per share)	$ (0.07)	$ 0.45	$ 0.80	$ 0.55
Diluted earnings (loss) per share (in dollars per share)	(0.07)	0.45	0.80	0.55
Dividends per share (in dollars per share)	$ 0.2675	$ 0.2625	$ 0.7975	$ 0.7825
Basic weighted average shares outstanding (in shares)	6,786	6,924	6,831	7,051
Diluted weighted average shares outstanding (in shares)	6,786	6,935	6,858	7,065