Summary of Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 214,585	$ 159,919	$ 149,540	$ 185,554	$ 139,204	$ 148,762	$ 156,822	$ 127,109	$ 709,598		$ 571,897		$ 561,407
Costs applicable to sales	121,982	101,559	102,229	114,490	84,903	84,594	81,820	84,058	440,260	[1]	335,375	[1]	403,827	[1]
Amortization	44,722	32,401	30,733	38,693	28,625	26,040	35,653	26,210	146,549		116,528		125,953
Exploration	7,454	9,792	7,813	5,252	5,260	3,706	2,233	1,731	30,311		12,930		11,521
Other Cost and Expense, Operating	15,106	12,374	11,110	13,962	10,370	10,439	10,688	16,635
Income (loss) from continuing operations	14,349	(11,728)	(9,995)	18,299	(10,328)	46,123	8,280	(21,640)	10,925		22,435		(287,811)
Income (loss) from discontinued operations.	(6,724)	(4,924)	(960)	364	2,020	23,436	6,217	1,244	(12,244)		32,917		(79,372)
Net income (loss)	7,625	(16,652)	(10,955)	18,663	(8,308)	69,559	14,497	(20,396)	(1,319)		55,352		(367,183)
Cash provided by (used in) operating activities	91,811	37,308	24,103	43,938	21,423	39,201	34,752	1,085	208,456		125,817		113,542
Capital expenditures	$ (47,054)	$ (28,982)	$ (37,107)	$ (23,591)	$ (28,134)	$ (22,626)	$ (21,971)	$ (21,651)	$ (136,734)		$ (94,382)		$ (88,973)
Basic net income (loss) from continuing operations (USD per share)	$ 0.08	$ (0.06)	$ (0.05)	$ 0.10	$ (0.06)	$ 0.29	$ 0.05	$ (0.15)	$ 0.06		$ 0.14		$ (2.22)
Basic net income (loss) from discontinued operations (USD per share)	(0.04)	(0.03)	(0.01)	0.00	0.01	0.14	0.04	0.01	(0.07)		0.21		(0.61)
Basic (in dollars per share)	0.04	(0.09)	(0.06)	0.10	(0.05)	0.43	0.09	(0.14)	(0.01)		0.35		(2.83)
Diluted net income (loss) from continuing operations (USD per share)	0.04	(0.09)	(0.06)	0.10	(0.05)	0.42	0.09	(0.14)	0.06		0.14		(2.22)
Diluted net income (loss) from discontinued operations (USD per share)	0.08	(0.06)	(0.05)	0.10	(0.06)	0.28	0.05	(0.15)	(0.07)		0.20		(0.61)
Diluted (in dollars per share)	$ (0.04)	$ (0.03)	$ (0.01)	$ 0.00	$ 0.01	$ 0.14	$ 0.04	$ 0.01	$ (0.01)		$ 0.34		$ (2.83)

[1] Excludes amortization.