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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	Mar. 31, 2015	Dec. 31, 2014
Assets:
Total assets	$ 254,062us-gaap_AssetsFairValueDisclosure	$ 316,249us-gaap_AssetsFairValueDisclosure
Money market funds [Member]
Assets:
Total assets	742	742
Commodity derivative instruments [Member]
Assets:
Total assets	253,320	315,507
Level 1 [Member]
Assets:
Total assets	742	742
Level 1 [Member] \| Money market funds [Member]
Assets:
Total assets	742	742
Level 2 [Member]
Assets:
Total assets	253,320	315,507
Level 2 [Member] \| Commodity derivative instruments [Member]
Assets:
Total assets	253,320	315,507
Level 3 [Member]
Assets:
Total assets	0	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