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Related party transactions and balances - Summary of Related Party Transactions (Detail) (Advertising [Member], USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Advertising [Member]
     
Related Party Transaction [Line Items]      
Revenue from provision of advertising services $ 32 $ 45 $ 55
Expenses for leases of advertising space     $ 643