Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $)	9 Months Ended
Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Maximum correlation between price of oil & natural gas in market and underlying price basis indicative in the derivative contract	100.00%
Accumulated other comprehensive income (loss)	$ 6,413,000	$ 24,500,000	$ 28,833,000
Accumulated other comprehensive income, to be reclassified into earning in the next twelve months	$ 3,800,000

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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