Derivative Financial Instruments - Details of Derivative Contracts (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Forward [Member]
Derivative [Line Items]
Date of transaction	Aug. 19, 2011
Type of derivative	Forward
Total notional amount	$ 54,000
Month of settlement	October to December 2012
Zero Cost Collar One [Member]
Derivative [Line Items]
Date of transaction	Mar. 23, 2012
Type of derivative	Zero cost collar
Total notional amount	54,000
Month of settlement	January to March 2013
Zero Cost Collar Two [Member]
Derivative [Line Items]
Date of transaction	May 18, 2012
Type of derivative	Zero cost collar
Total notional amount	$ 54,000
Month of settlement	April to June 2013