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Net loss per share (Details)
 1 Months Ended 3 Months Ended
Mar. 31, 2014
Mar. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share 13,343,922us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount 4,155,727us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Unvested restricted common stock sold to founders    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share 3,343,922us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_RestrictedStockMember		 2,964,502us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_RestrictedStockMember
Unvested restricted common stock issued upon early exercise of stock options    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= flks_NonvestedRestrictedCommonStockIssuedUponEarlyExerciseOfOptionsMember		 37,064us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= flks_NonvestedRestrictedCommonStockIssuedUponEarlyExerciseOfOptionsMember
Options to purchase common stock    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember		 1,154,161us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_EmployeeStockOptionMember
Series A preferred stock | Preferred stock    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Antidilutive securities excluded from computation of earnings per share 10,000,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_PreferredStockMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember		 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_PreferredStockMember
/ us-gaap_StatementClassOfStockAxis
= us-gaap_SeriesAPreferredStockMember