Derivative Instruments and Hedging Activities - Gains or Losses Related to Changes in Fair Value and Cash Settlements on Derivatives Not Qualifying as Hedging Instruments (Detail) (Commodity Contracts [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014
Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Cash settlements on Derivative Instruments not Qualifying Hedges	$ 1,484
Change in fair value on Derivative Instruments not Qualifying Hedges	12,146
Total gains on Derivative Instruments not Qualifying Hedges	$ 13,630