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	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Cash settlements	$ 17,385	$ 1,484
Change in fair value	(12,146)	12,146
Total gain on non-qualifying derivatives	$ 5,239	$ 13,630