Derivative Instruments - Fair Values of Derivative Contracts Recorded in Condensed Consolidated Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Document Period End Date	Jun. 30, 2014
Derivatives designated as fair value hedges, Asset derivatives	$ 136	$ 385
Derivatives not designated as hedging instruments, Liability derivatives	32	127
Cash Flow Hedge Derivative Instrument Liabilities at Fair Value	2
Mark-to-market energy assets [Member] | Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivatives asset not designated as hedging instruments, Forward contracts, Asset Derivatives	37	196
Mark-to-market energy assets [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivatives asset not designated as hedging instruments, Call Option, Derivatives assets	0	169
Mark-to-market energy assets [Member] | Put Option [Member] | Derivatives designated as fair value hedges [Member]
Derivatives, Fair Value [Line Items]
Derivatives designated as fair value hedges, Asset derivatives	99	20
Mark-to-market energy liabilities [Member] | Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivatives not designated as hedging instruments, Liability derivatives	$ 30	$ 127