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Line of credit - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
1 Months Ended
Mar. 31, 2014
Apr. 30, 2015
Line of Credit Facility [Line Items]    
Line of Credit Facility, Interest Rate During Period 3.00%us-gaap_LineOfCreditFacilityInterestRateDuringPeriod  
Line of Credit Facility, Expiration Date Jun. 30, 2016  
Letter of Credit    
Line of Credit Facility [Line Items]    
Line of Credit Facility, Amount Outstanding 5.0us-gaap_LineOfCredit
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LetterOfCreditMember
$ 0us-gaap_LineOfCredit
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LetterOfCreditMember