Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands					3 Months Ended									12 Months Ended
	Nov. 09, 2015	Jul. 24, 2015	May. 08, 2015	Feb. 12, 2015	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Sales					$ 136,112	$ 155,408	$ 147,511	$ 203,958	$ 249,589	$ 241,256	$ 205,801	$ 180,095	$ 545,985	$ 642,989	$ 876,741
Cost of goods sold (excluding depreciation, depletion and amortization)					116,614	122,599	117,200	138,653	157,700	149,697	132,417	126,770	348,567	495,066	566,584
Operating expenses
Selling, general and administrative					15,682	13,559	6,575	26,961	35,659	18,600	19,267	15,445	49,759	62,777	88,971
Depreciation, depletion and amortization					16,378	15,158	13,695	13,243	12,664	12,425	10,341	9,589	36,418	58,474	45,019	$ 36,418
Total operating expenses					32,060	28,717	20,270	40,204	48,323	31,025	29,608	25,034	86,177	121,251	133,990
Operating income					(12,562)	4,092	10,041	25,101	43,566	60,534	43,776	28,291	111,241	26,672	176,167
Other income (expense)
Interest expense					(6,835)	(6,684)	(6,928)	(6,836)	(5,431)	(4,950)	(4,013)	(3,808)	(15,341)	(27,283)	(18,202)
Early extinguishment of debt													480	0	0
Other income, net, including interest income					(90)	309	498	11	379	120	221	38	597	728	758
Total other (expenses) income					(6,925)	(6,375)	(6,430)	(6,825)	(5,052)	(4,830)	(3,792)	(3,770)	(15,224)	(26,555)	(17,444)
Income before income taxes					(19,487)	(2,283)	3,611	18,276	38,514	55,704	39,984	24,521	96,017	117	158,723
Income tax benefit (expense)					4,167	4,695	6,342	(3,453)	(5,276)	(14,427)	(11,330)	(6,150)	(20,761)	11,751	(37,183)
Net income					$ (15,320)	$ 2,412	$ 9,953	$ 14,823	$ 33,238	$ 41,277	$ 28,654	$ 18,371	$ 75,256	$ 11,868	$ 121,540	$ 75,256
Earnings per share:
Basic (in usd per share)					$ (0.29)	$ 0.05	$ 0.19	$ 0.28	$ 0.62	$ 0.77	$ 0.53	$ 0.34	$ 1.42	$ 0.22	$ 2.26
Diluted (in usd per share)					$ (0.29)	$ 0.04	$ 0.18	$ 0.27	$ 0.61	$ 0.76	$ 0.53	$ 0.34	1.41	$ 0.22	$ 2.24
Weighted Average Number of Shares Outstanding, Basic					53,323	53,321	53,303	53,416	53,838	53,801	53,733	53,505		53,344	53,719	53,035
Weighted Average Number of Shares Outstanding, Diluted														53,601	54,296	53,409
Quarterly cash dividend declared	$ 0.0625	$ 0.125	$ 0.125	$ 0.125	$ 0.0625	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.125	$ 0.375	$ 0.4375	$ 0.50	$ 0.38