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Line of credit - Additional Information (Detail) - USD ($)
$ in Millions
1 Months Ended
Mar. 31, 2014
Jul. 31, 2015
Line of Credit Facility [Line Items]    
Line of Credit Facility, Interest Rate During Period 3.00%  
Line of Credit Facility, Expiration Date Jun. 30, 2016  
Letter of Credit    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Amount Outstanding $ 5.0 $ 0.0