Summary of significant accounting policies - Components of Changes in Accumulated Other Comprehensive Income (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]
Beginning Balance	$ 3,714
Unrealized gain (loss) on cross-currency swaps, net of tax	263	(437)	(90)
Foreign currency translation adjustment	768	(2,279)	(1,317)
Ending Balance	4,745	3,714
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	3,846	6,125
Foreign currency translation adjustment	768	(2,279)
Ending Balance	4,614	3,846
Fair Value of Cross- Currency Swaps [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(132)	305
Unrealized gain (loss) on cross-currency swaps, net of tax	263	(437)
Foreign currency translation adjustment
Ending Balance	131	(132)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	3,714	6,430
Unrealized gain (loss) on cross-currency swaps, net of tax	263	(437)	(90)
Foreign currency translation adjustment	768	(2,279)	(1,317)
Ending Balance	$ 4,745	$ 3,714	$ 6,430