Mammoth Complex Acquisition - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended						5 Months Ended	7 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		Dec. 31, 2010 Mammoth Pacific	Aug. 01, 2010 Mammoth Pacific	Dec. 31, 2010 Mammoth Pacific	Dec. 31, 2010 Mammoth Pacific Minimum acre	Dec. 31, 2010 Mammoth Pacific Maximum
Business Acquisition [Line Items]
Acquired ownership interest																50.00%
Business acquisition purchase price in cash																$ 72,500,000
Ownership interest prior to acquisition																50.00%
Undeveloped federal lands																		10,000
Fair value of previously held equity interest																64,900,000
Control premium of previously held equity interest																7,600,000
Gain on acquisition of controlling interest													36,928,000				36,900,000
Percentage of weighted average cost of capital																		9.50%	14.00%
Revenues	116,082,000	136,137,000	129,837,000	132,352,000	123,734,000	110,841,000	104,614,000	97,820,000	514,408,000	[1]	437,009,000	[1]	373,230,000	[1]	7,567,000
Net income (loss)	$ (222,384,000)	$ (582,000)	$ 8,641,000	$ 7,895,000	$ (43,080,000)	$ 866,000	$ 8,133,000	$ (8,975,000)	$ (206,430,000)		$ (43,056,000)		$ 37,318,000		$ 645,000	$ 998,000

[1] Revenues as reported in the geographic area in which they originate.