Derivative Instruments - Fair Values of Derivative Contracts Recorded in Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Derivative Asset Fair Value Net	$ 210	$ 1,754
Derivatives liability not designated as hedging instruments	331	1,496
Mark-to-market energy assets [Member] | Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivatives asset not designated as hedging instruments	182	1,686
Mark-to-market energy assets [Member] | Put Option [Member] | Derivatives designated as fair value hedges [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset Fair Value Net		68
Mark-to-market energy assets [Member] | Call options [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset Fair Value Net	28
Mark-to-market energy liabilities [Member] | Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivatives liability not designated as hedging instruments	$ 331	$ 1,496