Accumulated Other Comprehensive Income (Loss) - Schedule of Changes in Accumulated Other Comprehensive Income Loss (Detail) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	$ 83,315,000	$ (2,100,000)
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	22,900,000	(17,400,000)
Foreign currency translations	(3,600,000)	(500,000)
Income tax effect	(8,200,000)us-gaap_OtherComprehensiveIncomeLossTax	6,300,000us-gaap_OtherComprehensiveIncomeLossTax
Net of tax	11,100,000	(11,600,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	23,600,000	(4,500,000)
Other comprehensive income (loss), net of tax	(12,500,000)	(7,100,000)
Ending balance, net of tax	70,812,000	(9,200,000)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	36,800,000	(7,100,000)
Income tax effect	(13,200,000)	2,600,000
Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	86,800,000	(1,400,000)
Other comprehensive income (loss) before reclassifications:
Change in fair value of derivatives	22,900,000	(17,400,000)
Income tax effect	(8,200,000)	6,300,000
Net of tax	14,700,000	(11,100,000)
Amounts reclassified from accumulated other comprehensive income:
Net of tax	23,600,000	(4,500,000)
Other comprehensive income (loss), net of tax	(8,900,000)	(6,600,000)
Ending balance, net of tax	77,900,000	(8,000,000)
Cash Flow Hedges [Member] \| Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from accumulated other comprehensive income:
Operating revenue: oil/gas production	36,800,000	(7,100,000)
Income tax effect	(13,200,000)	2,600,000
Foreign Currency Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	(3,500,000)	(700,000)
Other comprehensive income (loss) before reclassifications:
Foreign currency translations	(3,600,000)	(500,000)
Net of tax	(3,600,000)	(500,000)
Amounts reclassified from accumulated other comprehensive income:
Other comprehensive income (loss), net of tax	(3,600,000)	(500,000)
Ending balance, net of tax	$ (7,100,000)	$ (1,200,000)

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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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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Amount before tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income of accumulated gain (loss) realized from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's deferred hedging gain (loss).

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Amount of tax expense (benefit) allocated to other comprehensive income (loss).

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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