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Common stock - Narrative (Details) (USD $)
1 Months Ended 10 Months Ended
Apr. 30, 2014
Mar. 31, 2014
Dec. 31, 2014
Class of Stock [Line Items]      
Common stock, shares authorized     61,000,000us-gaap_CommonStockSharesAuthorized
Common stock, par value (in usd per share)     $ 0.0001us-gaap_CommonStockParOrStatedValuePerShare
Proceeds from sale of restricted common stock to founders     $ 2,321flks_ProceedsFromSaleOfRestrictedCommonStock
Unvested restricted common stock      
Class of Stock [Line Items]      
Restricted common stock, shares sold 867,314us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
4,553,415us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Restricted common stock, price per share (in usd per share)   $ 0.0004us-gaap_SaleOfStockPricePerShare
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Proceeds from sale of restricted common stock to founders   1,950flks_ProceedsFromSaleOfRestrictedCommonStock
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
 
Restricted common stock, repurchase period     90 days
Restricted common stock, repurchase price     $ 0.0004us-gaap_TreasuryStockAcquiredAverageCostPerShare
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Restricted common stock, shares outstanding     2,202,139us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Restricted common stock, fair value of shares vested     $ 3,620,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
Vests upon issuance | Unvested restricted common stock      
Class of Stock [Line Items]      
Restricted common stock, percent vested     25.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= us-gaap_ShareBasedCompensationAwardTrancheOneMember
Vests ratably over four years | Unvested restricted common stock      
Class of Stock [Line Items]      
Restricted common stock, percent vested     75.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockMember
/ us-gaap_VestingAxis
= us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Restricted common stock, vesting period     4 years