Summary of significant accounting policies - Schedule of Components of and 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	$ 1,761
Foreign currency translation adjustment	480	(3,192)	(1,710)
Ending Balance	2,504	1,761
Unrealized gain (loss) on cross-currency swaps, net of tax	263	(437)	(90)
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1,893	5,085
Foreign currency translation adjustment	480	(3,192)
Ending Balance	2,373	1,893
Unrealized gain (loss) on cross-currency swaps, net of tax
Fair Value of Cross- Currency Swaps [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	(132)	305
Foreign currency translation adjustment
Ending Balance	131	(132)
Unrealized gain (loss) on cross-currency swaps, net of tax	263	(437)
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning Balance	1,761	5,390
Foreign currency translation adjustment	480	(3,192)	(1,710)
Ending Balance	2,504	1,761	5,390
Unrealized gain (loss) on cross-currency swaps, net of tax	$ 263	$ (437)	$ (90)