Fair Value Measurements - Reconciliation of the Net Beginning and Ending Balances for Derivative Instruments (Detail) (Fair Value, Inputs, Level 3, USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Fair Value, Inputs, Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Beginning balance	$ 548
Changes in fair value of derivative instruments	2,979
Settlements received	$ (3,527)

Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as an asset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

Name:	us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsAssetAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant