3.20.1 html 255 481 1 false 90 0 false 9 false false R1.htm 100000 - Document - Document and Entity Information Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DocumentDocumentAndEntityInformation Document and Entity Information Cover 1 false false R2.htm 100010 - Statement - Consolidated Balance Sheets Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedBalanceSheets Consolidated Balance Sheets Statements 2 false false R3.htm 100020 - Statement - Consolidated Balance Sheets (Parenthetical) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical Consolidated Balance Sheets (Parenthetical) Statements 3 false false R4.htm 100030 - Statement - Consolidated Statements of Operations and Comprehensive Loss Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss Consolidated Statements of Operations and Comprehensive Loss Statements 4 false false R5.htm 100040 - Statement - Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLossParenthetical Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) Statements 5 false false R6.htm 100050 - Statement - Consolidated Statements of Changes in Stockholders' (Deficit) Equity Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfChangesInStockholdersDeficitEquity Consolidated Statements of Changes in Stockholders' (Deficit) Equity Statements 6 false false R7.htm 100060 - Statement - Consolidated Statements of Cash Flows Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows Consolidated Statements of Cash Flows Statements 7 false false R8.htm 100070 - Disclosure - Nature of the Business Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNatureOfBusiness Nature of the Business Notes 8 false false R9.htm 100080 - Disclosure - Financing Transactions Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFinancingTransactions Financing Transactions Notes 9 false false R10.htm 100090 - Disclosure - Summary of Significant Accounting Policies Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies Summary of Significant Accounting Policies Notes 10 false false R11.htm 100100 - Disclosure - Fair Value of Financial Assets and Liabilities Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilities Fair Value of Financial Assets and Liabilities Notes 11 false false R12.htm 100110 - Disclosure - Marketable Securities Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureMarketableSecurities Marketable Securities Notes 12 false false R13.htm 100120 - Disclosure - Prepaid Expenses and Other Current Assets Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssets Prepaid Expenses and Other Current Assets Notes 13 false false R14.htm 100130 - Disclosure - Inventory Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureInventory Inventory Notes 14 false false R15.htm 100140 - Disclosure - Property and Equipment, Net Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNet Property and Equipment, Net Notes 15 false false R16.htm 100150 - Disclosure - Accrued Expenses and Other Current Liabilities Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities Accrued Expenses and Other Current Liabilities Notes 16 false false R17.htm 100160 - Disclosure - Debt Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebt Debt Notes 17 false false R18.htm 100170 - Disclosure - Stockholders' Equity Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquity Stockholders' Equity Notes 18 false false R19.htm 100180 - Disclosure - Stock Plans Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockPlans Stock Plans Notes 19 false false R20.htm 100190 - Disclosure - Stock-Based Compensation Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensation Stock-Based Compensation Notes 20 false false R21.htm 100200 - Disclosure - Commitments and Contingencies Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingencies Commitments and Contingencies Notes 21 false false R22.htm 100210 - Disclosure - Net Loss Per Share Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShare Net Loss Per Share Notes 22 false false R23.htm 100220 - Disclosure - Income Taxes Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxes Income Taxes Notes 23 false false R24.htm 100230 - Disclosure - Quarterly Financial Data (unaudited) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureQuarterlyFinancialDataUnaudited Quarterly Financial Data (unaudited) Notes 24 false false R25.htm 100240 - Disclosure - Summary of Significant Accounting Policies (Policies) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies Summary of Significant Accounting Policies (Policies) Policies http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies 25 false false R26.htm 100250 - Disclosure - Summary of Significant Accounting Policies (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesTables Summary of Significant Accounting Policies (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies 26 false false R27.htm 100260 - Disclosure - Fair Value of Financial Assets and Liabilities (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesTables Fair Value of Financial Assets and Liabilities (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilities 27 false false R28.htm 100270 - Disclosure - Marketable Securities (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureMarketableSecuritiesTables Marketable Securities (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureMarketableSecurities 28 false false R29.htm 100280 - Disclosure - Prepaid Expenses and Other Current Assets (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssetsTables Prepaid Expenses and Other Current Assets (Tables) Tables 29 false false R30.htm 100290 - Disclosure - Inventory (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureInventoryTables Inventory (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureInventory 30 false false R31.htm 100300 - Disclosure - Property and Equipment, Net (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetTables Property and Equipment, Net (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNet 31 false false R32.htm 100310 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables Accrued Expenses and Other Current Liabilities (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities 32 false false R33.htm 100320 - Disclosure - Debt (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebtTables Debt (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebt 33 false false R34.htm 100330 - Disclosure - Stock-Based Compensation (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationTables Stock-Based Compensation (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensation 34 false false R35.htm 100340 - Disclosure - Commitments and Contingencies (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesTables Commitments and Contingencies (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingencies 35 false false R36.htm 100350 - Disclosure - Net Loss Per Share (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareTables Net Loss Per Share (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShare 36 false false R37.htm 100360 - Disclosure - Income Taxes (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesTables Income Taxes (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxes 37 false false R38.htm 100370 - Disclosure - Quarterly Financial Data (unaudited) (Tables) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureQuarterlyFinancialDataUnauditedTables Quarterly Financial Data (unaudited) (Tables) Tables http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureQuarterlyFinancialDataUnaudited 38 false false R39.htm 100380 - Disclosure - Nature of the Business - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNatureOfBusinessAdditionalInformationDetail Nature of the Business - Additional Information (Detail) Details 39 false false R40.htm 100390 - Disclosure - Financing Transactions - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFinancingTransactionsAdditionalInformationDetail Financing Transactions - Additional Information (Detail) Details 40 false false R41.htm 100400 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail Summary of Significant Accounting Policies - Additional Information (Detail) Details 41 false false R42.htm 100410 - Disclosure - Summary of Significant Accounting Policies - Summary of Product Revenue Allowance And Reserve Categories (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesSummaryOfProductRevenueAllowanceAndReserveCategoriesDetail Summary of Significant Accounting Policies - Summary of Product Revenue Allowance And Reserve Categories (Detail) Details 42 false false R43.htm 100420 - Disclosure - Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPropertyPlantAndEquipmentEstimatedUsefulLivesDetail Summary of Significant Accounting Policies - Property Plant and Equipment Estimated Useful Lives (Detail) Details 43 false false R44.htm 100430 - Disclosure - Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesScheduleOfAssetsMeasuredAtFairValueOnRecurringBasisDetail Fair Value of Financial Assets and Liabilities - Schedule of Assets Measured at Fair Value on Recurring Basis (Detail) Details 44 false false R45.htm 100440 - Disclosure - Fair Value of Financial Assets and Liabilities - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesAdditionalInformationDetail Fair Value of Financial Assets and Liabilities - Additional Information (Detail) Details 45 false false R46.htm 100450 - Disclosure - Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureMarketableSecuritiesSummaryOfFairValueOfAvailableForSaleMarketableSecuritiesByTypeOfSecurityDetail Marketable Securities - Summary of Fair Value of Available-for-Sale Marketable Securities by Type of Security (Detail) Details 46 false false R47.htm 100460 - Disclosure - Marketable Securities - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureMarketableSecuritiesAdditionalInformationDetail Marketable Securities - Additional Information (Detail) Details 47 false false R48.htm 100470 - Disclosure - Prepaid Expenses and Other Current Assets - Prepaid Expenses and Other Current Assets (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssetsPrepaidExpensesAndOtherCurrentAssetsDetail Prepaid Expenses and Other Current Assets - Prepaid Expenses and Other Current Assets (Detail) Details 48 false false R49.htm 100480 - Disclosure - Inventory - Schedule of Inventory (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureInventoryScheduleOfInventoryDetail Inventory - Schedule of Inventory (Detail) Details 49 false false R50.htm 100490 - Disclosure - Inventory - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureInventoryAdditionalInformationDetail Inventory - Additional Information (Detail) Details 50 false false R51.htm 100500 - Disclosure - Property and Equipment, Net - Components of Property and Equipment (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetComponentsOfPropertyAndEquipmentDetail Property and Equipment, Net - Components of Property and Equipment (Detail) Details 51 false false R52.htm 100510 - Disclosure - Property and Equipment, Net - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetAdditionalInformationDetail Property and Equipment, Net - Additional Information (Detail) Details 52 false false R53.htm 100520 - Disclosure - Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesScheduleOfAccruedExpensesAndOtherCurrentLiabilitiesDetail Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) Details 53 false false R54.htm 100530 - Disclosure - Debt - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebtAdditionalInformationDetail Debt - Additional Information (Detail) Details 54 false false R55.htm 100540 - Disclosure - Debt - Schedule of Annual Principal and Interest Payments Due Under Term Loan (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebtScheduleOfAnnualPrincipalAndInterestPaymentsDueUnderTermLoanDetail Debt - Schedule of Annual Principal and Interest Payments Due Under Term Loan (Detail) Details 55 false false R56.htm 100550 - Disclosure - Debt - Summary of Carrying Value of Convertible Notes (Detail) Notes http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureDebtSummaryOfCarryingValueOfConvertibleNotesDetail Debt - Summary of Carrying Value of Convertible Notes (Detail) Details 56 false false R57.htm 100560 - Disclosure - Stockholders' Equity - Additional information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityAdditionalInformationDetail Stockholders' Equity - Additional information (Detail) Details 57 false false R58.htm 100570 - Disclosure - Stock Plans - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockPlansAdditionalInformationDetail Stock Plans - Additional Information (Detail) Details 58 false false R59.htm 100580 - Disclosure - Stock-Based Compensation - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationAdditionalInformationDetail Stock-Based Compensation - Additional Information (Detail) Details 59 false false R60.htm 100590 - Disclosure - Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationScheduleOfRelevantDataUsedToEstimateFairValueOfStockOptionGrantsDetail Stock-Based Compensation - Schedule of Relevant Data Used to Estimate Fair Value of Stock Option Grants (Detail) Details 60 false false R61.htm 100600 - Disclosure - Stock-Based Compensation - Summary of Stock Option Activity (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationSummaryOfStockOptionActivityDetail Stock-Based Compensation - Summary of Stock Option Activity (Detail) Details 61 false false R62.htm 100610 - Disclosure - Stock-Based Compensation - Summary of of RSU Activity (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationSummaryOfOfRSUActivityDetail Stock-Based Compensation - Summary of of RSU Activity (Detail) Details 62 false false R63.htm 100620 - Disclosure - Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureStockBasedCompensationStockBasedCompensationExpenseRelatedToStockOptionsRestrictedStockAndSharesPurchasedUnderEmployeeStockPurchasePlanDetail Stock-Based Compensation - Stock-Based Compensation Expense Related to Stock Options, Restricted Stock and Shares Purchased Under Employee Stock Purchase Plan (Detail) Details 63 false false R64.htm 100630 - Disclosure - Commitments and Contingencies - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetail Commitments and Contingencies - Additional Information (Detail) Details 64 false false R65.htm 100640 - Disclosure - Commitments and Contingencies - Components of Lease Expense and Related Cash Flows (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesComponentsOfLeaseExpenseAndRelatedCashFlowsDetail Commitments and Contingencies - Components of Lease Expense and Related Cash Flows (Detail) Details 65 false false R66.htm 100650 - Disclosure - Commitments and Contingencies - Schedule of Maturities of Lease Liability Due Under Lease Agreements (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfMaturitiesOfLeaseLiabilityDueUnderLeaseAgreementsDetail Commitments and Contingencies - Schedule of Maturities of Lease Liability Due Under Lease Agreements (Detail) Details 66 false false R67.htm 100660 - Disclosure - Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesFutureMinimumLeasePaymentsUnderOperatingLeasesDetail Commitments and Contingencies - Future Minimum Lease Payments under Operating Leases (Detail) Details 67 false false R68.htm 100670 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Payments under Manufacturing Agreement with Patheon (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumPaymentsUnderManufacturingAgreementWithPatheonDetail Commitments and Contingencies - Schedule of Future Minimum Payments under Manufacturing Agreement with Patheon (Detail) Details 68 false false R69.htm 100680 - Disclosure - Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareScheduleOfBasicAndDilutedNetLossPerShareAttributableToCommonStockholdersDetail Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) Details 69 false false R70.htm 100690 - Disclosure - Net Loss per Share - Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareScheduleOfCommonStockEquivalentsExcludedFromCalculationOfDilutedNetLossPerShareAsIncludingThemWouldHaveAntiDilutiveEffectDetail Net Loss per Share - Schedule of Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share, as Including them Would have Anti-dilutive Effect (Detail) Details 70 false false R71.htm 100700 - Disclosure - Income Taxes - Additional Information (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail Income Taxes - Additional Information (Detail) Details 71 false false R72.htm 100710 - Disclosure - Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetail Income Taxes - Reconciliation of U.S. Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail) Details 72 false false R73.htm 100720 - Disclosure - Income Taxes - Net Deferred Tax Assets (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesNetDeferredTaxAssetsDetail Income Taxes - Net Deferred Tax Assets (Detail) Details 73 false false R74.htm 100730 - Disclosure - Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesChangesInValuationAllowanceForDeferredTaxAssetsDetail Income Taxes - Changes in Valuation Allowance for Deferred Tax Assets (Detail) Details 74 false false R75.htm 100740 - Disclosure - Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) Sheet http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureQuarterlyFinancialDataUnauditedRecurringAdjustmentsNecessaryForFairStatementDetail Quarterly Financial Data (unaudited) - Recurring Adjustments Necessary for Fair Statement (Detail) Details http://flexiontherapeutics.com/20191231/taxonomy/role/DisclosureQuarterlyFinancialDataUnauditedTables 75 false false All Reports Book All Reports flxn-10k_20191231.htm flxn-20191231.xsd flxn-20191231_cal.xml flxn-20191231_def.xml flxn-20191231_lab.xml flxn-20191231_pre.xml flxn-ex1015_43.htm flxn-ex106_42.htm flxn-ex211_11.htm flxn-ex231_8.htm flxn-ex311_6.htm flxn-ex312_10.htm flxn-ex321_9.htm flxn-ex322_7.htm flxn-ex45_41.htm gyqh3yzy551h000001.jpg gyqh3yzy551h000002.jpg http://fasb.org/us-gaap/2019-01-31 http://xbrl.sec.gov/dei/2019-01-31 http://fasb.org/srt/2019-01-31 http://xbrl.sec.gov/stpr/2018-01-31 true true