Oil and Gas Producing Activities - Costs Incurred Related to Oil and Gas Property Acquisition Exploration and Development Activities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property Acquisitions:
Unproved property acquisitions	$ 91,960	$ 130,113	$ 13,742
Proved property acquisitions	2,400	6,471
Development costs	440,848us-gaap_CostsIncurredDevelopmentCosts	341,970us-gaap_CostsIncurredDevelopmentCosts	331,254us-gaap_CostsIncurredDevelopmentCosts
Exploration costs	52,080us-gaap_CostsIncurredExplorationCosts	439us-gaap_CostsIncurredExplorationCosts	5,522us-gaap_CostsIncurredExplorationCosts
Costs incurred, acquisition of oil and gas properties, total	$ 587,288	$ 478,993	$ 350,518

Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have proved reserves.

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Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves.

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Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

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