3.19.1 html 276 443 1 false 106 0 false 7 false false R1.htm 100000 - Document - Document and Entity Information Sheet http://www.mtem.com/20181231/taxonomy/role/DocumentDocumentAndEntityInformation Document and Entity Information Cover 1 false false R2.htm 100010 - Statement - Consolidated Balance Sheets Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedBalanceSheets Consolidated Balance Sheets Statements 2 false false R3.htm 100020 - Statement - Consolidated Balance Sheets (Parenthetical) Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical Consolidated Balance Sheets (Parenthetical) Statements 3 false false R4.htm 100030 - Statement - Consolidated Statements of Operations and Comprehensive Loss Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss Consolidated Statements of Operations and Comprehensive Loss Statements 4 false false R5.htm 100040 - Statement - Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficit Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) Statements 5 false false R6.htm 100050 - Statement - Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedStatementsOfConvertiblePreferredStockAndStockholdersEquityDeficitParenthetical Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) Statements 6 false false R7.htm 100060 - Statement - Consolidated Statements of Cash Flows Sheet http://www.mtem.com/20181231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows Consolidated Statements of Cash Flows Statements 7 false false R8.htm 100070 - Disclosure - Summary of Significant Accounting Policies Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies Summary of Significant Accounting Policies Notes 8 false false R9.htm 100080 - Disclosure - Net Loss Per Common Share Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShare Net Loss Per Common Share Notes 9 false false R10.htm 100090 - Disclosure - Merger with Private Molecular Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecular Merger with Private Molecular Notes 10 false false R11.htm 100100 - Disclosure - Research and Development Agreements Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreements Research and Development Agreements Notes 11 false false R12.htm 100110 - Disclosure - Marketable Securities and Fair Value Measurements Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurements Marketable Securities and Fair Value Measurements Notes 12 false false R13.htm 100120 - Disclosure - Property and Equipment Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosurePropertyAndEquipment Property and Equipment Notes 13 false false R14.htm 100130 - Disclosure - Balance Sheet Components Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBalanceSheetComponents Balance Sheet Components Notes 14 false false R15.htm 100140 - Disclosure - Related Party Transactions Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRelatedPartyTransactions Related Party Transactions Notes 15 false false R16.htm 100150 - Disclosure - Borrowing Arrangements Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBorrowingArrangements Borrowing Arrangements Notes 16 false false R17.htm 100160 - Disclosure - Commitments and Contingencies Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingencies Commitments and Contingencies Notes 17 false false R18.htm 100170 - Disclosure - Redeemable Convertible Preferred Stock Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStock Redeemable Convertible Preferred Stock Notes 18 false false R19.htm 100180 - Disclosure - Stockholders' Equity Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquity Stockholders' Equity Notes 19 false false R20.htm 100190 - Disclosure - Equity Incentive Plans and Stock Based Compensation Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensation Equity Incentive Plans and Stock Based Compensation Notes 20 false false R21.htm 100200 - Disclosure - Income Taxes Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxes Income Taxes Notes 21 false false R22.htm 100210 - Disclosure - Employee Benefit Plan Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEmployeeBenefitPlan Employee Benefit Plan Notes 22 false false R23.htm 100220 - Disclosure - Subsequent Events Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureSubsequentEvents Subsequent Events Notes 23 false false R24.htm 100230 - Disclosure - Summary of Significant Accounting Policies (Policies) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies Summary of Significant Accounting Policies (Policies) Policies http://www.mtem.com/20181231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies 24 false false R25.htm 100240 - Disclosure - Net Loss Per Common Share (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShareTables Net Loss Per Common Share (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShare 25 false false R26.htm 100250 - Disclosure - Merger with Private Molecular (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularTables Merger with Private Molecular (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecular 26 false false R27.htm 100260 - Disclosure - Research and Development Agreements (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsTables Research and Development Agreements (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreements 27 false false R28.htm 100270 - Disclosure - Marketable Securities and Fair Value Measurements (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurementsTables Marketable Securities and Fair Value Measurements (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurements 28 false false R29.htm 100280 - Disclosure - Property and Equipment (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosurePropertyAndEquipmentTables Property and Equipment (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosurePropertyAndEquipment 29 false false R30.htm 100290 - Disclosure - Balance Sheet Components (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBalanceSheetComponentsTables Balance Sheet Components (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureBalanceSheetComponents 30 false false R31.htm 100300 - Disclosure - Borrowing Arrangements (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBorrowingArrangementsTables Borrowing Arrangements (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureBorrowingArrangements 31 false false R32.htm 100310 - Disclosure - Commitments and Contingencies (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingenciesTables Commitments and Contingencies (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingencies 32 false false R33.htm 100320 - Disclosure - Redeemable Convertible Preferred Stock (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockTables Redeemable Convertible Preferred Stock (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStock 33 false false R34.htm 100330 - Disclosure - Stockholders' Equity (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquityTables Stockholders' Equity (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquity 34 false false R35.htm 100340 - Disclosure - Equity Incentive Plans and Stock Based Compensation (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationTables Equity Incentive Plans and Stock Based Compensation (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensation 35 false false R36.htm 100350 - Disclosure - Income Taxes (Tables) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxesTables Income Taxes (Tables) Tables http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxes 36 false false R37.htm 100360 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail Summary of Significant Accounting Policies - Additional Information (Detail) Details 37 false false R38.htm 100370 - Disclosure - Net Loss Per Common Share - Reconciliation of Numerator and Denominator (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShareReconciliationOfNumeratorAndDenominatorDetail Net Loss Per Common Share - Reconciliation of Numerator and Denominator (Detail) Details 38 false false R39.htm 100380 - Disclosure - Net Loss Per Common Share - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShareAdditionalInformationDetail Net Loss Per Common Share - Additional Information (Detail) Details 39 false false R40.htm 100390 - Disclosure - Net Loss Per Common Share - Warrants and Options Split Adjusted, and Excluded from Computation of Diluted Net Loss Per Share (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureNetLossPerCommonShareWarrantsAndOptionsSplitAdjustedAndExcludedFromComputationOfDilutedNetLossPerShareDetail Net Loss Per Common Share - Warrants and Options Split Adjusted, and Excluded from Computation of Diluted Net Loss Per Share (Detail) Details 40 false false R41.htm 100400 - Disclosure - Merger with Private Molecular - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularAdditionalInformationDetail Merger with Private Molecular - Additional Information (Detail) Details 41 false false R42.htm 100410 - Disclosure - Merger with Private Molecular - Purchase Price for Threshold on Closing Date of Merger (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularPurchasePriceForThresholdOnClosingDateOfMergerDetail Merger with Private Molecular - Purchase Price for Threshold on Closing Date of Merger (Detail) Details 42 false false R43.htm 100420 - Disclosure - Merger with Private Molecular - Purchase Price for Threshold on Closing Date of Merger (Parenthetical) (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularPurchasePriceForThresholdOnClosingDateOfMergerParentheticalDetail Merger with Private Molecular - Purchase Price for Threshold on Closing Date of Merger (Parenthetical) (Detail) Details 43 false false R44.htm 100430 - Disclosure - Merger with Private Molecular - Summary of Allocation of Purchase Consideration to Assets Acquired and Liabilities Assumed (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularSummaryOfAllocationOfPurchaseConsiderationToAssetsAcquiredAndLiabilitiesAssumedDetail Merger with Private Molecular - Summary of Allocation of Purchase Consideration to Assets Acquired and Liabilities Assumed (Detail) Details 44 false false R45.htm 100440 - Disclosure - Merger with Private Molecular - Pro Forma Results in Connection with Merger (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMergerWithPrivateMolecularProFormaResultsInConnectionWithMergerDetail Merger with Private Molecular - Pro Forma Results in Connection with Merger (Detail) Details 45 false false R46.htm 100450 - Disclosure - Research and Development Agreements - Schedule of Research and Development Revenues Disaggregated by Location (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsScheduleOfResearchAndDevelopmentRevenuesDisaggregatedByLocationDetail Research and Development Agreements - Schedule of Research and Development Revenues Disaggregated by Location (Detail) Details 46 false false R47.htm 100460 - Disclosure - Research and Development Agreements - Schedule of Condensed Consolidated Balance Sheet (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsScheduleOfCondensedConsolidatedBalanceSheetDetail Research and Development Agreements - Schedule of Condensed Consolidated Balance Sheet (Detail) Details 47 false false R48.htm 100470 - Disclosure - Research and Development Agreements - Schedule of Contract Assets and Contract Liabilities for Performance Obligations related to Collaboration Agreements (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsScheduleOfContractAssetsAndContractLiabilitiesForPerformanceObligationsRelatedToCollaborationAgreementsDetail Research and Development Agreements - Schedule of Contract Assets and Contract Liabilities for Performance Obligations related to Collaboration Agreements (Detail) Details 48 false false R49.htm 100480 - Disclosure - Research and Development Agreements - Schedule of Contract Assets and Contract Liabilities for Performance Obligations related to Collaboration Agreements (Parenthetical) (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsScheduleOfContractAssetsAndContractLiabilitiesForPerformanceObligationsRelatedToCollaborationAgreementsParentheticalDetail Research and Development Agreements - Schedule of Contract Assets and Contract Liabilities for Performance Obligations related to Collaboration Agreements (Parenthetical) (Detail) Details 49 false false R50.htm 100490 - Disclosure - Research and Development Agreements - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureResearchAndDevelopmentAgreementsAdditionalInformationDetail Research and Development Agreements - Additional Information (Detail) Details 50 false false R51.htm 100500 - Disclosure - Marketable Securities and Fair Value Measurements - Financial Assets at Fair Value on Recurring Basis (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurementsFinancialAssetsAtFairValueOnRecurringBasisDetail Marketable Securities and Fair Value Measurements - Financial Assets at Fair Value on Recurring Basis (Detail) Details 51 false false R52.htm 100510 - Disclosure - Marketable Securities and Fair Value Measurements - Summary of Company's Available-for-Sale Securities (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurementsSummaryOfCompanySAvailableForSaleSecuritiesDetail Marketable Securities and Fair Value Measurements - Summary of Company's Available-for-Sale Securities (Detail) Details 52 false false R53.htm 100520 - Disclosure - Marketable Securities and Fair Value Measurements - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurementsAdditionalInformationDetail Marketable Securities and Fair Value Measurements - Additional Information (Detail) Details 53 false false R54.htm 100530 - Disclosure - Marketable Securities and Fair Value Measurements - Financial Liabilities at Fair Value on Recurring Basis (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureMarketableSecuritiesAndFairValueMeasurementsFinancialLiabilitiesAtFairValueOnRecurringBasisDetail Marketable Securities and Fair Value Measurements - Financial Liabilities at Fair Value on Recurring Basis (Detail) Details 54 false false R55.htm 100540 - Disclosure - Property and Equipment - Components of Property and Equipment (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosurePropertyAndEquipmentComponentsOfPropertyAndEquipmentDetail Property and Equipment - Components of Property and Equipment (Detail) Details 55 false false R56.htm 100550 - Disclosure - Property and Equipment - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosurePropertyAndEquipmentAdditionalInformationDetail Property and Equipment - Additional Information (Detail) Details 56 false false R57.htm 100560 - Disclosure - Balance Sheet Components - Components of Accrued Liabilities (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBalanceSheetComponentsComponentsOfAccruedLiabilitiesDetail Balance Sheet Components - Components of Accrued Liabilities (Detail) Details 57 false false R58.htm 100570 - Disclosure - Balance Sheet Components - Components of Deferred Revenue (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBalanceSheetComponentsComponentsOfDeferredRevenueDetail Balance Sheet Components - Components of Deferred Revenue (Detail) Details 58 false false R59.htm 100580 - Disclosure - Related Party Transactions - Additional Information (Details) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails Related Party Transactions - Additional Information (Details) Details 59 false false R60.htm 100590 - Disclosure - Borrowing Arrangements - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBorrowingArrangementsAdditionalInformationDetail Borrowing Arrangements - Additional Information (Detail) Details 60 false false R61.htm 100600 - Disclosure - Borrowing Arrangements - Schedule of Required Future Principal Payments (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureBorrowingArrangementsScheduleOfRequiredFuturePrincipalPaymentsDetail Borrowing Arrangements - Schedule of Required Future Principal Payments (Detail) Details 61 false false R62.htm 100610 - Disclosure - Commitments and Contingencies - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetail Commitments and Contingencies - Additional Information (Detail) Details 62 false false R63.htm 100620 - Disclosure - Commitments and Contingencies - Future Minimum Payments Due under Operating Lease Agreements (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingenciesFutureMinimumPaymentsDueUnderOperatingLeaseAgreementsDetail Commitments and Contingencies - Future Minimum Payments Due under Operating Lease Agreements (Detail) Details 63 false false R64.htm 100630 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Capital Lease Payments (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumCapitalLeasePaymentsDetail Commitments and Contingencies - Schedule of Future Minimum Capital Lease Payments (Detail) Details 64 false false R65.htm 100650 - Disclosure - Redeemable Convertible Preferred Stock - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockAdditionalInformationDetail Redeemable Convertible Preferred Stock - Additional Information (Detail) Details 65 false false R66.htm 100660 - Disclosure - Redeemable Convertible Preferred Stock - Schedule of Changes in the Preferred Stock (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockScheduleOfChangesInPreferredStockDetail Redeemable Convertible Preferred Stock - Schedule of Changes in the Preferred Stock (Detail) Details 66 false false R67.htm 100670 - Disclosure - Stockholders' Equity - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquityAdditionalInformationDetail Stockholders' Equity - Additional Information (Detail) Details 67 false false R68.htm 100680 - Disclosure - Stockholders' Equity - Outstanding Warrants to Purchase Shares of Common Stock (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquityOutstandingWarrantsToPurchaseSharesOfCommonStockDetail Stockholders' Equity - Outstanding Warrants to Purchase Shares of Common Stock (Detail) Details 68 false false R69.htm 100690 - Disclosure - Stockholders' Equity - Reconciliation of Warrant Liability Measured at Fair Value (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquityReconciliationOfWarrantLiabilityMeasuredAtFairValueDetail Stockholders' Equity - Reconciliation of Warrant Liability Measured at Fair Value (Detail) Details 69 false false R70.htm 100700 - Disclosure - Stockholders' Equity - Outstanding Warrants Valuation Assumption (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureStockholdersEquityOutstandingWarrantsValuationAssumptionDetail Stockholders' Equity - Outstanding Warrants Valuation Assumption (Detail) Details 70 false false R71.htm 100710 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationAdditionalInformationDetail Equity Incentive Plans and Stock Based Compensation - Additional Information (Detail) Details 71 false false R72.htm 100720 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Summary of Stock Option Activity Under Equity Incentive Plan (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfStockOptionActivityUnderEquityIncentivePlanDetail Equity Incentive Plans and Stock Based Compensation - Summary of Stock Option Activity Under Equity Incentive Plan (Detail) Details 72 false false R73.htm 100730 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Summary of Stock Option Activity Under Equity Incentive Plan (Parenthetical) (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationSummaryOfStockOptionActivityUnderEquityIncentivePlanParentheticalDetail Equity Incentive Plans and Stock Based Compensation - Summary of Stock Option Activity Under Equity Incentive Plan (Parenthetical) (Detail) Details 73 false false R74.htm 100740 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Stock Options Outstanding and Exercisable by Exercise Price (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationStockOptionsOutstandingAndExercisableByExercisePriceDetail Equity Incentive Plans and Stock Based Compensation - Stock Options Outstanding and Exercisable by Exercise Price (Detail) Details 74 false false R75.htm 100750 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Stock-Based Compensation Expense (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationStockBasedCompensationExpenseDetail Equity Incentive Plans and Stock Based Compensation - Stock-Based Compensation Expense (Detail) Details 75 false false R76.htm 100760 - Disclosure - Equity Incentive Plans and Stock Based Compensation - Weighted-Average Fair Value Valuation Assumptions (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEquityIncentivePlansAndStockBasedCompensationWeightedAverageFairValueValuationAssumptionsDetail Equity Incentive Plans and Stock Based Compensation - Weighted-Average Fair Value Valuation Assumptions (Detail) Details 76 false false R77.htm 100770 - Disclosure - Income Taxes - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail Income Taxes - Additional Information (Detail) Details 77 false false R78.htm 100780 - Disclosure - Income Taxes - Schedule of Reconciliation of Income Taxes at Statutory Federal Income Tax Rate to Net Income Taxes (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxesScheduleOfReconciliationOfIncomeTaxesAtStatutoryFederalIncomeTaxRateToNetIncomeTaxesDetail Income Taxes - Schedule of Reconciliation of Income Taxes at Statutory Federal Income Tax Rate to Net Income Taxes (Detail) Details 78 false false R79.htm 100790 - Disclosure - Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxesScheduleOfSignificantComponentsOfNetDeferredTaxAssetsDetail Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Detail) Details 79 false false R80.htm 100800 - Disclosure - Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureIncomeTaxesScheduleOfActivityRelatedToUnrecognizedTaxBenefitsDetail Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) Details 80 false false R81.htm 100810 - Disclosure - Employee Benefit Plan - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureEmployeeBenefitPlanAdditionalInformationDetail Employee Benefit Plan - Additional Information (Detail) Details 81 false false R82.htm 100820 - Disclosure - Subsequent Events - Additional Information (Detail) Sheet http://www.mtem.com/20181231/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetail Subsequent Events - Additional Information (Detail) Details 82 false false All Reports Book All Reports mtem-20181231.xml mtem-20181231.xsd mtem-20181231_cal.xml mtem-20181231_def.xml mtem-20181231_lab.xml mtem-20181231_pre.xml http://fasb.org/us-gaap/2018-01-31 http://xbrl.sec.gov/country/2017-01-31 http://xbrl.sec.gov/stpr/2018-01-31 http://xbrl.sec.gov/dei/2018-01-31 http://fasb.org/srt/2018-01-31 true true