Fair Value Measurements - Hierarchy of Fair Value, Assets and Liabilities Measured on Recurring Basis (Detail) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Assets:
Total assets	$ 41,038	$ 4,339
Liabilities:
Total liabilities		8,327
Money market funds [Member]
Assets:
Total assets	742	742
Commodity derivative instruments [Member]
Assets:
Total assets	40,296	3,597
Liabilities:
Total liabilities		8,327
Level 1 [Member]
Assets:
Total assets	742	742
Liabilities:
Total liabilities
Level 1 [Member] | Money market funds [Member]
Assets:
Total assets	742	742
Level 1 [Member] | Commodity derivative instruments [Member]
Assets:
Total assets
Liabilities:
Total liabilities
Level 2 [Member]
Assets:
Total assets	40,296	3,597
Liabilities:
Total liabilities		8,327
Level 2 [Member] | Money market funds [Member]
Assets:
Total assets
Level 2 [Member] | Commodity derivative instruments [Member]
Assets:
Total assets	40,296	3,597
Liabilities:
Total liabilities		8,327
Level 3 [Member]
Assets:
Total assets	0	0
Liabilities:
Total liabilities		0
Level 3 [Member] | Money market funds [Member]
Assets:
Total assets	0	0
Level 3 [Member] | Commodity derivative instruments [Member]
Assets:
Total assets	0	0
Liabilities:
Total liabilities		$ 0