Non-GAAP Financial Measures - Summary of Reconciliation of Non-GAAP measure (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating income	$ 4,618	$ 8,073	$ 7,368	$ 8,092	$ 12,315	$ 9,534	$ 11,905	$ 11,740	$ 28,151	$ 45,494	$ 45,728
Plus: Depreciation, amortization, accretion and impairment									16,677	15,167	21,614
EBITDA (as defined by the Company)									44,828	60,661	67,342
Less: Purchases of property and equipment									(7,679)	(10,408)	(9,989)
OCF (as defined by the Company)									$ 37,149	$ 50,253	$ 57,353