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Subsequent events - Additional Information (Detail) (Subsequent Event [Member], USD $)
0 Months Ended
Feb. 06, 2015
Subsequent Event [Line Items]  
Restricted stock closing price awarded to executive officers $ 40.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Restricted Stock [Member] | Executive officers [Member]
 
Subsequent Event [Line Items]  
Total restricted shares issued to executive officers 49,955us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
/ us-gaap_TitleOfIndividualAxis
= us-gaap_ExecutiveOfficerMember