Quarterly Data (Quarterly Earnings Data) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income									$ 54,416	$ 144,295	$ 50,365
Revenue	203,374	181,044	212,775	203,051	259,651	253,652	222,605	170,061	800,244	905,969	555,878
Depletion, depreciation and amortization	14,374	14,107	14,862	14,672	15,027	15,628	15,660	16,063	58,015	62,378	71,593
Operating income	18,597	9,645	13,627	1,767	20,994	44,156	49,117	40,939	43,636	155,206	54,712
Net income	16,876	12,803	18,797	5,940	24,664	40,741	42,698	36,192	55,045	144,295	50,365
Comprehensive income attributable to common stockholders	17,116	13,497	19,046	6,248	24,894	40,506	42,784	36,002	55,907	144,186	49,603
Basic earnings per share attributable to common stockholders	$ 0.14	$ 0.11	$ 0.17	$ 0.05	$ 0.21	$ 0.40	$ 0.41	$ 0.35	$ 0.47	$ 1.36	$ 0.51
Diluted earnings per share attributable to common stockholders	$ 0.13	$ 0.11	$ 0.17	$ 0.05	$ 0.21	$ 0.37	$ 0.39	$ 0.34	$ 0.46	$ 1.30	$ 0.51
Previously Reported [Member]
Revenue		181,044	212,775	203,051
Depletion, depreciation and amortization		14,107	14,862	14,672
Operating income		9,645	13,627	1,767
Net income		12,692	17,829	2,439
Comprehensive income attributable to common stockholders		13,417	18,091	2,747
Basic earnings per share attributable to common stockholders		$ 0.11	$ 0.16	$ 0.02
Diluted earnings per share attributable to common stockholders		$ 0.11	$ 0.15	$ 0.02
Restatement Adjustment [Member]
Net income		$ 100	$ 1,000	$ 3,500