Deferred Revenue - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013 T	Dec. 31, 2012
Deferred Revenue Arrangement [Line Items]
Proceeds from sale of future production	$ 62,000	$ 73,583,000
Deferred revenue recognized during the period	253,000
Volumetric production payment termination date	Dec. 31, 2052
Volumetric production payment percentage sold	10.50%
Volumetric production payment estimated tons to be delivered	15,000,000
Minimum
Deferred Revenue Arrangement [Line Items]
Estimated deferred revenue to be recognized in the next 12 months	1,000,000
Maximum
Deferred Revenue Arrangement [Line Items]
Estimated deferred revenue to be recognized in the next 12 months	$ 1,400,000
Tons subject to volumetric production payment	143,200,000