Acquisition of Working Interests in Natural Gas Properties - Summary of Unaudited Pro forma Information During Period of Acquisition (Detail) (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
Business Acquisition Actual Revenue And Pre Tax Income Loss [Line Items]
Net sales-as reported	$ 177,137	$ 182,374	$ 209,275	$ 190,245	$ 215,364	$ 176,780	$ 235,619	$ 177,493	$ 759,031	$ 805,256	$ 609,905
Net sales-pro forma									764,362	808,710
Income from continuing operations-as reported	11,622	6,710	26,130	14,324	28,002	6,324	28,698	20,960	58,786	83,984	29,715
Income from continuing operations-pro forma									$ 60,750	$ 84,956
Basic income per common share-as reported	$ 0.52	$ 0.30	$ 1.17	$ 0.63	$ 1.26	$ 0.28	$ 1.29	$ 0.97	$ 2.61	$ 3.80	$ 1.38
Basic income per common share-pro forma									$ 2.70	$ 3.85
Diluted income per common share-as reported	$ 0.49	$ 0.28	$ 1.11	$ 0.61	$ 1.19	$ 0.27	$ 1.22	$ 0.90	$ 2.49	$ 3.58	$ 1.32
Diluted income per common share-pro forma									$ 2.57	$ 3.62