Costs Incurred for Property Acquisition, Exploration and Development (Parenthetical) (Detail) (Conger Exchange Transaction, USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
Apr. 30, 2014	Dec. 31, 2014
Conger Exchange Transaction
Costs Incurred Oil And Gas Property Acquisition Exploration And Development Activities [Line Items]
Cash proceeds from sale of assets	$ 145.0	$ 145.0

Aggregate cash proceeds received from a combination of transactions in which noncurrent assets are sold, which may include the sale of a business, an investment in an affiliate (including an equity method investee), property, plant and equipment and intangible assets. Excludes sales of trading, available-for-sale, and held-to-maturity securities.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration