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Common stock - Shares reserved for future issuance (Details)
Dec. 31, 2014
Class of Stock [Line Items]  
Stock-based compensation awards 1,189,314flks_ShareBasedCompensationAwardSharesReservedForFutureIssuance
Total 11,379,012flks_SharesReservedForFutureIssuance
Series A Preferred Stock  
Class of Stock [Line Items]  
Convertible Preferred Stock 3,683,637us-gaap_ConvertiblePreferredStockSharesReservedForFutureIssuance
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember
Series B Preferred Stock  
Class of Stock [Line Items]  
Convertible Preferred Stock 3,287,471us-gaap_ConvertiblePreferredStockSharesReservedForFutureIssuance
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesBPreferredStockMember
Unvested restricted common stock  
Class of Stock [Line Items]  
Vesting of restricted common stock 3,218,590us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember