Summary of significant accounting policies - Components of Changes in Accumulated Other Comprehensive Income (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on cross-currency swaps, tax (benefit)	$ 153	$ 256	$ 36
Fair Value of Cross- Currency Swaps [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on cross-currency swaps, tax (benefit)	153	(256)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized gain (loss) on cross-currency swaps, tax (benefit)	$ 153	$ (256)