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Financial instruments - Fair Value Amounts of Derivative Instruments and Their Effect on Statement of Operations (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Ineffective portion of derivative included in income	$ 0	$ 0

The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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