Summary of significant accounting policies - Schedule of Components of and 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 loss on cross-currency swaps, tax benefit	$ 153	$ 256	$ 36
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized loss on cross-currency swaps, tax benefit	153	256
Fair Value of Cross- Currency Swaps [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized loss on cross-currency swaps, tax benefit
Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized loss on cross-currency swaps, tax benefit