3.20.1 html 505 830 1 false 124 0 false 9 false false R1.htm 00090 - Document - Document and Entity Information Sheet http://www.harborone.com/role/DocumentDocumentAndEntityInformation Document and Entity Information Cover 1 false false R2.htm 00100 - Statement - Consolidated Balance Sheets Sheet http://www.harborone.com/role/StatementConsolidatedBalanceSheets Consolidated Balance Sheets Statements 2 false false R3.htm 00105 - Statement - Consolidated Balance Sheets (Parenthetical) Sheet http://www.harborone.com/role/StatementConsolidatedBalanceSheetsParenthetical Consolidated Balance Sheets (Parenthetical) Statements 3 false false R4.htm 00200 - Statement - Consolidated Statements of Operations Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfOperations Consolidated Statements of Operations Statements 4 false false R5.htm 00205 - Statement - Consolidated Statements of Operations (Parenthetical) Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfOperationsParenthetical Consolidated Statements of Operations (Parenthetical) Statements 5 false false R6.htm 00300 - Statement - Consolidated Statements of Comprehensive Income Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfComprehensiveIncome Consolidated Statements of Comprehensive Income Statements 6 false false R7.htm 00305 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical) Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfComprehensiveIncomeParenthetical Consolidated Statements of Comprehensive Income (Parenthetical) Statements 7 false false R8.htm 00400 - Statement - Consolidated Statements of Changes in Stockholders' Equity Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquity Consolidated Statements of Changes in Stockholders' Equity Statements 8 false false R9.htm 00405 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityParenthetical Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) Statements 9 false false R10.htm 00500 - Statement - Consolidated Statements of Cash Flows Sheet http://www.harborone.com/role/StatementConsolidatedStatementsOfCashFlows Consolidated Statements of Cash Flows Statements 10 false false R11.htm 10101 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Sheet http://www.harborone.com/role/DisclosureSummaryOfSignificantAccountingPolicies SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Notes 11 false false R12.htm 10201 - Disclosure - BUSINESS COMBINATIONS Sheet http://www.harborone.com/role/DisclosureBusinessCombinations BUSINESS COMBINATIONS Notes 12 false false R13.htm 10301 - Disclosure - CASH AND DUE FROM BANKS Sheet http://www.harborone.com/role/DisclosureCashAndDueFromBanks CASH AND DUE FROM BANKS Notes 13 false false R14.htm 10401 - Disclosure - SECURITIES Sheet http://www.harborone.com/role/DisclosureSecurities SECURITIES Notes 14 false false R15.htm 10501 - Disclosure - LOANS HELD FOR SALE Sheet http://www.harborone.com/role/DisclosureLoansHeldForSale LOANS HELD FOR SALE Notes 15 false false R16.htm 10601 - Disclosure - LOANS Sheet http://www.harborone.com/role/DisclosureLoans LOANS Notes 16 false false R17.htm 10701 - Disclosure - MORTGAGE LOAN SERVICING Sheet http://www.harborone.com/role/DisclosureMortgageLoanServicing MORTGAGE LOAN SERVICING Notes 17 false false R18.htm 10801 - Disclosure - OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS Sheet http://www.harborone.com/role/DisclosureOtherRealEstateLoansAndRepossessedAssets OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS Notes 18 false false R19.htm 10901 - Disclosure - PROPERTY AND EQUIPMENT Sheet http://www.harborone.com/role/DisclosurePropertyAndEquipment PROPERTY AND EQUIPMENT Notes 19 false false R20.htm 11001 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS Sheet http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssets GOODWILL AND OTHER INTANGIBLE ASSETS Notes 20 false false R21.htm 11101 - Disclosure - DEPOSITS Sheet http://www.harborone.com/role/DisclosureDeposits DEPOSITS Notes 21 false false R22.htm 11201 - Disclosure - FHLB BORROWINGS Sheet http://www.harborone.com/role/DisclosureFhlbBorrowings FHLB BORROWINGS Notes 22 false false R23.htm 11301 - Disclosure - SUBORDINATED DEBENTURES Sheet http://www.harborone.com/role/DisclosureSubordinatedDebentures SUBORDINATED DEBENTURES Notes 23 false false R24.htm 11401 - Disclosure - INCOME TAXES Sheet http://www.harborone.com/role/DisclosureIncomeTaxes INCOME TAXES Notes 24 false false R25.htm 11501 - Disclosure - OTHER COMMITMENTS AND CONTINGENCIES Sheet http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingencies OTHER COMMITMENTS AND CONTINGENCIES Notes 25 false false R26.htm 11601 - Disclosure - DERIVATIVES Sheet http://www.harborone.com/role/DisclosureDerivatives DERIVATIVES Notes 26 false false R27.htm 11701 - Disclosure - COMPENSATION AND BENEFIT PLANS Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlans COMPENSATION AND BENEFIT PLANS Notes 27 false false R28.htm 11801 - Disclosure - STOCK-BASED COMPENSATION Sheet http://www.harborone.com/role/DisclosureStockBasedCompensation STOCK-BASED COMPENSATION Notes 28 false false R29.htm 11901 - Disclosure - STOCKHOLDERS' EQUITY Sheet http://www.harborone.com/role/DisclosureStockholdersEquity STOCKHOLDERS' EQUITY Notes 29 false false R30.htm 12001 - Disclosure - COMPREHENSIVE INCOME (LOSS) Sheet http://www.harborone.com/role/DisclosureComprehensiveIncomeLoss COMPREHENSIVE INCOME (LOSS) Notes 30 false false R31.htm 12101 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilities FAIR VALUE OF ASSETS AND LIABILITIES Notes 31 false false R32.htm 12201 - Disclosure - EARNINGS PER SHARE (???EPS???) Sheet http://www.harborone.com/role/DisclosureEarningsPerShareEps EARNINGS PER SHARE (???EPS???) Notes 32 false false R33.htm 12301 - Disclosure - SEGMENT REPORTING Sheet http://www.harborone.com/role/DisclosureSegmentReporting SEGMENT REPORTING Notes 33 false false R34.htm 12401 - Disclosure - CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY Sheet http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompany CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY Notes 34 false false R35.htm 12501 - Disclosure - SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) Sheet http://www.harborone.com/role/DisclosureSelectedQuarterlyFinancialDataUnaudited SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) Notes 35 false false R36.htm 12601 - Disclosure - SUBSEQUENT EVENT Sheet http://www.harborone.com/role/DisclosureSubsequentEvent SUBSEQUENT EVENT Notes 36 false false R37.htm 20102 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies) Sheet http://www.harborone.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies) Policies 37 false false R38.htm 30203 - Disclosure - BUSINESS COMBINATION (Tables) Sheet http://www.harborone.com/role/DisclosureBusinessCombinationTables BUSINESS COMBINATION (Tables) Tables http://www.harborone.com/role/DisclosureBusinessCombinations 38 false false R39.htm 30403 - Disclosure - SECURITIES (Tables) Sheet http://www.harborone.com/role/DisclosureSecuritiesTables SECURITIES (Tables) Tables http://www.harborone.com/role/DisclosureSecurities 39 false false R40.htm 30503 - Disclosure - LOANS HELD FOR SALE (Tables) Sheet http://www.harborone.com/role/DisclosureLoansHeldForSaleTables LOANS HELD FOR SALE (Tables) Tables http://www.harborone.com/role/DisclosureLoansHeldForSale 40 false false R41.htm 30603 - Disclosure - LOANS (Tables) Sheet http://www.harborone.com/role/DisclosureLoansTables LOANS (Tables) Tables http://www.harborone.com/role/DisclosureLoansHeldForSale 41 false false R42.htm 30703 - Disclosure - MORTGAGE LOAN SERVICING (Tables) Sheet http://www.harborone.com/role/DisclosureMortgageLoanServicingTables MORTGAGE LOAN SERVICING (Tables) Tables http://www.harborone.com/role/DisclosureMortgageLoanServicing 42 false false R43.htm 30803 - Disclosure - OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS (Table) Sheet http://www.harborone.com/role/DisclosureOtherRealEstateLoansAndRepossessedAssetsTable OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS (Table) Tables http://www.harborone.com/role/DisclosureOtherRealEstateLoansAndRepossessedAssets 43 false false R44.htm 30903 - Disclosure - PROPERTY AND EQUIPMENT (Tables) Sheet http://www.harborone.com/role/DisclosurePropertyAndEquipmentTables PROPERTY AND EQUIPMENT (Tables) Tables http://www.harborone.com/role/DisclosurePropertyAndEquipment 44 false false R45.htm 31003 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS (Tables) Sheet http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssetsTables GOODWILL AND OTHER INTANGIBLE ASSETS (Tables) Tables http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssets 45 false false R46.htm 31103 - Disclosure - DEPOSITS (Tables) Sheet http://www.harborone.com/role/DisclosureDepositsTables DEPOSITS (Tables) Tables http://www.harborone.com/role/DisclosureDeposits 46 false false R47.htm 31203 - Disclosure - FHLB BORROWING (Tables) Sheet http://www.harborone.com/role/DisclosureFhlbBorrowingTables FHLB BORROWING (Tables) Tables http://www.harborone.com/role/DisclosureFhlbBorrowings 47 false false R48.htm 31403 - Disclosure - INCOME TAXES (Tables) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesTables INCOME TAXES (Tables) Tables http://www.harborone.com/role/DisclosureIncomeTaxes 48 false false R49.htm 31503 - Disclosure - OTHER COMMITMENTS AND CONTINGENCIES (Tables) Sheet http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingenciesTables OTHER COMMITMENTS AND CONTINGENCIES (Tables) Tables http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingencies 49 false false R50.htm 31603 - Disclosure - DERIVATIVES (Tables) Sheet http://www.harborone.com/role/DisclosureDerivativesTables DERIVATIVES (Tables) Tables http://www.harborone.com/role/DisclosureDerivatives 50 false false R51.htm 31703 - Disclosure - COMPENSATION AND BENEFIT PLANS (Tables) Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansTables COMPENSATION AND BENEFIT PLANS (Tables) Tables http://www.harborone.com/role/DisclosureCompensationAndBenefitPlans 51 false false R52.htm 31803 - Disclosure - STOCK-BASED COMPENSATION (Tables) Sheet http://www.harborone.com/role/DisclosureStockBasedCompensationTables STOCK-BASED COMPENSATION (Tables) Tables http://www.harborone.com/role/DisclosureStockBasedCompensation 52 false false R53.htm 31903 - Disclosure - STOCKHOLDERS??? EQUITY (Tables) Sheet http://www.harborone.com/role/DisclosureStockholdersEquityTables STOCKHOLDERS??? EQUITY (Tables) Tables 53 false false R54.htm 32003 - Disclosure - COMPREHENSIVE INCOME (LOSS) (Tables) Sheet http://www.harborone.com/role/DisclosureComprehensiveIncomeLossTables COMPREHENSIVE INCOME (LOSS) (Tables) Tables http://www.harborone.com/role/DisclosureComprehensiveIncomeLoss 54 false false R55.htm 32103 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES (Tables) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesTables FAIR VALUE OF ASSETS AND LIABILITIES (Tables) Tables http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilities 55 false false R56.htm 32203 - Disclosure - EARNINGS PER SHARE (Tables) Sheet http://www.harborone.com/role/DisclosureEarningsPerShareTables EARNINGS PER SHARE (Tables) Tables http://www.harborone.com/role/DisclosureEarningsPerShareEps 56 false false R57.htm 32303 - Disclosure - SEGMENT REPORTING (Tables) Sheet http://www.harborone.com/role/DisclosureSegmentReportingTables SEGMENT REPORTING (Tables) Tables http://www.harborone.com/role/DisclosureSegmentReporting 57 false false R58.htm 32403 - Disclosure - CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY (Tables) Sheet http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompanyTables CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY (Tables) Tables http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompany 58 false false R59.htm 32503 - Disclosure - SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables) Sheet http://www.harborone.com/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables) Tables http://www.harborone.com/role/DisclosureSelectedQuarterlyFinancialDataUnaudited 59 false false R60.htm 40101 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) Sheet http://www.harborone.com/role/DisclosureSummaryOfSignificantAccountingPoliciesDetails SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) Details http://www.harborone.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies 60 false false R61.htm 40102 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) Sheet http://www.harborone.com/role/DisclosureSummaryOfSignificantAccountingPoliciesRecentAccountingPronouncementsDetails SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) Details 61 false false R62.htm 40201 - Disclosure - BUSINESS COMBINATIONS - Acquisition of Coastway Bancorp, Inc. (Details) Sheet http://www.harborone.com/role/DisclosureBusinessCombinationsAcquisitionOfCoastwayBancorpIncDetails BUSINESS COMBINATIONS - Acquisition of Coastway Bancorp, Inc. (Details) Details 62 false false R63.htm 40301 - Disclosure - CASH AND DUE FROM BANKS (Details) Sheet http://www.harborone.com/role/DisclosureCashAndDueFromBanksDetails CASH AND DUE FROM BANKS (Details) Details http://www.harborone.com/role/DisclosureCashAndDueFromBanks 63 false false R64.htm 40401 - Disclosure - SECURITIES - Gross unrealized gains and losses (Details) Sheet http://www.harborone.com/role/DisclosureSecuritiesGrossUnrealizedGainsAndLossesDetails SECURITIES - Gross unrealized gains and losses (Details) Details 64 false false R65.htm 40402 - Disclosure - SECURITIES - Contractual maturity (Details) Sheet http://www.harborone.com/role/DisclosureSecuritiesContractualMaturityDetails SECURITIES - Contractual maturity (Details) Details 65 false false R66.htm 40403 - Disclosure - SECURITIES - Gross unrealized losses aggregated by category (Details) Sheet http://www.harborone.com/role/DisclosureSecuritiesGrossUnrealizedLossesAggregatedByCategoryDetails SECURITIES - Gross unrealized losses aggregated by category (Details) Details 66 false false R67.htm 40501 - Disclosure - LOANS HELD FOR SALE (Details) Sheet http://www.harborone.com/role/DisclosureLoansHeldForSaleDetails LOANS HELD FOR SALE (Details) Details http://www.harborone.com/role/DisclosureLoansHeldForSaleTables 67 false false R68.htm 40601 - Disclosure - LOANS - Summary of Balances of Loans (Details) Sheet http://www.harborone.com/role/DisclosureLoansSummaryOfBalancesOfLoansDetails LOANS - Summary of Balances of Loans (Details) Details 68 false false R69.htm 40602 - Disclosure - LOANS - Loans Sold or Transferred (Details) Sheet http://www.harborone.com/role/DisclosureLoansLoansSoldOrTransferredDetails LOANS - Loans Sold or Transferred (Details) Details 69 false false R70.htm 40603 - Disclosure - LOANS - Acquired Loans (Details) Sheet http://www.harborone.com/role/DisclosureLoansAcquiredLoansDetails LOANS - Acquired Loans (Details) Details 70 false false R71.htm 40604 - Disclosure - LOANS - PCI Loans (Details) Sheet http://www.harborone.com/role/DisclosureLoansPciLoansDetails LOANS - PCI Loans (Details) Details 71 false false R72.htm 40605 - Disclosure - LOANS - Allowance for Loan Losses Activity and Allocation to Loan Segments (Details) Sheet http://www.harborone.com/role/DisclosureLoansAllowanceForLoanLossesActivityAndAllocationToLoanSegmentsDetails LOANS - Allowance for Loan Losses Activity and Allocation to Loan Segments (Details) Details 72 false false R73.htm 40606 - Disclosure - LOANS - Summary of Past Due and Non-Accrual Loans (Details) Sheet http://www.harborone.com/role/DisclosureLoansSummaryOfPastDueAndNonAccrualLoansDetails LOANS - Summary of Past Due and Non-Accrual Loans (Details) Details 73 false false R74.htm 40607 - Disclosure - LOANS - Impaired Loans (Details) Sheet http://www.harborone.com/role/DisclosureLoansImpairedLoansDetails LOANS - Impaired Loans (Details) Details 74 false false R75.htm 40608 - Disclosure - LOANS - Troubled Debt Restructurings (Details) Sheet http://www.harborone.com/role/DisclosureLoansTroubledDebtRestructuringsDetails LOANS - Troubled Debt Restructurings (Details) Details 75 false false R76.htm 40609 - Disclosure - LOANS - Risk Rating (Details) Sheet http://www.harborone.com/role/DisclosureLoansRiskRatingDetails LOANS - Risk Rating (Details) Details 76 false false R77.htm 40701 - Disclosure - MORTGAGE LOAN SERVICING - Key Assumptions (Details) Sheet http://www.harborone.com/role/DisclosureMortgageLoanServicingKeyAssumptionsDetails MORTGAGE LOAN SERVICING - Key Assumptions (Details) Details 77 false false R78.htm 40702 - Disclosure - MORTGAGE LOAN SERVICING - Fair value of MSR (Details) Sheet http://www.harborone.com/role/DisclosureMortgageLoanServicingFairValueOfMsrDetails MORTGAGE LOAN SERVICING - Fair value of MSR (Details) Details 78 false false R79.htm 40801 - Disclosure - OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS (Details) Sheet http://www.harborone.com/role/DisclosureOtherRealEstateLoansAndRepossessedAssetsDetails OTHER REAL ESTATE LOANS AND REPOSSESSED ASSETS (Details) Details http://www.harborone.com/role/DisclosureOtherRealEstateLoansAndRepossessedAssetsTable 79 false false R80.htm 40901 - Disclosure - PROPERTY AND EQUIPMENT (Details) Sheet http://www.harborone.com/role/DisclosurePropertyAndEquipmentDetails PROPERTY AND EQUIPMENT (Details) Details http://www.harborone.com/role/DisclosurePropertyAndEquipmentTables 80 false false R81.htm 41001 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Goodwill (Details) Sheet http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssetsGoodwillDetails GOODWILL AND OTHER INTANGIBLE ASSETS - Goodwill (Details) Details 81 false false R82.htm 41002 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Intangible Assets (Details) Sheet http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssetsIntangibleAssetsDetails GOODWILL AND OTHER INTANGIBLE ASSETS - Intangible Assets (Details) Details 82 false false R83.htm 41003 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS - Other Intangible Assets (Details) Sheet http://www.harborone.com/role/DisclosureGoodwillAndOtherIntangibleAssetsOtherIntangibleAssetsDetails GOODWILL AND OTHER INTANGIBLE ASSETS - Other Intangible Assets (Details) Details 83 false false R84.htm 41101 - Disclosure - DEPOSITS - Summary of deposits (Details) Sheet http://www.harborone.com/role/DisclosureDepositsSummaryOfDepositsDetails DEPOSITS - Summary of deposits (Details) Details 84 false false R85.htm 41102 - Disclosure - DEPOSITS - Maturity of deposits (Details) Sheet http://www.harborone.com/role/DisclosureDepositsMaturityOfDepositsDetails DEPOSITS - Maturity of deposits (Details) Details 85 false false R86.htm 41201 - Disclosure - FHLB BORROWINGS - FHLB Advances (Details) Sheet http://www.harborone.com/role/DisclosureFhlbBorrowingsFhlbAdvancesDetails FHLB BORROWINGS - FHLB Advances (Details) Details 86 false false R87.htm 41202 - Disclosure - FHLB BORROWINGS - Others (Details) Sheet http://www.harborone.com/role/DisclosureFhlbBorrowingsOthersDetails FHLB BORROWINGS - Others (Details) Details 87 false false R88.htm 41301 - Disclosure - SUBORDINATED DEBENTURES (Details) Sheet http://www.harborone.com/role/DisclosureSubordinatedDebenturesDetails SUBORDINATED DEBENTURES (Details) Details http://www.harborone.com/role/DisclosureSubordinatedDebentures 88 false false R89.htm 41401 - Disclosure - INCOME TAXES (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesDetails INCOME TAXES (Details) Details http://www.harborone.com/role/DisclosureIncomeTaxesTables 89 false false R90.htm 41402 - Disclosure - INCOME TAXES - Differences Between Federal Income Tax And Actual Income Tax (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesDifferencesBetweenFederalIncomeTaxAndActualIncomeTaxDetails INCOME TAXES - Differences Between Federal Income Tax And Actual Income Tax (Details) Details 90 false false R91.htm 41403 - Disclosure - INCOME TAXES - Deferred taxes (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesDeferredTaxesDetails INCOME TAXES - Deferred taxes (Details) Details 91 false false R92.htm 41404 - Disclosure - INCOME TAXES - Changes In Net Deferred Tax Asset (Liability) (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesChangesInNetDeferredTaxAssetLiabilityDetails INCOME TAXES - Changes In Net Deferred Tax Asset (Liability) (Details) Details 92 false false R93.htm 41405 - Disclosure - INCOME TAXES - Carryforwards (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesCarryforwardsDetails INCOME TAXES - Carryforwards (Details) Details 93 false false R94.htm 41406 - Disclosure - INCOME TAXES - UInrecognized Tax Benefits (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesUinrecognizedTaxBenefitsDetails INCOME TAXES - UInrecognized Tax Benefits (Details) Details 94 false false R95.htm 41407 - Disclosure - INCOME TAXES - Unrecognized tax benefits (Details) Sheet http://www.harborone.com/role/DisclosureIncomeTaxesUnrecognizedTaxBenefitsDetails INCOME TAXES - Unrecognized tax benefits (Details) Details 95 false false R96.htm 41601 - Disclosure - DERIVATIVES (Details) Sheet http://www.harborone.com/role/DisclosureDerivativesDetails DERIVATIVES (Details) Details http://www.harborone.com/role/DisclosureDerivativesTables 96 false false R97.htm 41602 - Disclosure - OTHER COMMITMENTS AND CONTINGENCIES - Contingent Consideration (Details) Sheet http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingenciesContingentConsiderationDetails OTHER COMMITMENTS AND CONTINGENCIES - Contingent Consideration (Details) Details 97 false false R98.htm 41603 - Disclosure - OTHER COMMITMENTS AND CONTINGENCIES (Details) Sheet http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingenciesDetails OTHER COMMITMENTS AND CONTINGENCIES (Details) Details http://www.harborone.com/role/DisclosureOtherCommitmentsAndContingenciesTables 98 false false R99.htm 41701 - Disclosure - COMPENSATION AND BENEFIT PLANS (Details) Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansDetails COMPENSATION AND BENEFIT PLANS (Details) Details http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansTables 99 false false R100.htm 41702 - Disclosure - COMPENSATION AND BENEFIT PLANS - Net Periodic Cost (Details) Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansNetPeriodicCostDetails COMPENSATION AND BENEFIT PLANS - Net Periodic Cost (Details) Details 100 false false R101.htm 41703 - Disclosure - COMPENSATION AND BENEFIT PLANS - Assumptions Used to Determine Benefit Obligation and Net Periodic Cost (Details) Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansAssumptionsUsedToDetermineBenefitObligationAndNetPeriodicCostDetails COMPENSATION AND BENEFIT PLANS - Assumptions Used to Determine Benefit Obligation and Net Periodic Cost (Details) Details 101 false false R102.htm 41704 - Disclosure - COMPENSATION AND BENEFIT PLANS - Expected Benefit Payments (Details) Sheet http://www.harborone.com/role/DisclosureCompensationAndBenefitPlansExpectedBenefitPaymentsDetails COMPENSATION AND BENEFIT PLANS - Expected Benefit Payments (Details) Details 102 false false R103.htm 41801 - Disclosure - STOCK-BASED COMPENSATION (Details) Sheet http://www.harborone.com/role/DisclosureStockBasedCompensationDetails STOCK-BASED COMPENSATION (Details) Details http://www.harborone.com/role/DisclosureStockBasedCompensationTables 103 false false R104.htm 41802 - Disclosure - STOCK-BASED COMPENSATION - Valuation assumptions (Details) Sheet http://www.harborone.com/role/DisclosureStockBasedCompensationValuationAssumptionsDetails STOCK-BASED COMPENSATION - Valuation assumptions (Details) Details 104 false false R105.htm 41803 - Disclosure - STOCK-BASED COMPENSATION - Stock options (Details) Sheet http://www.harborone.com/role/DisclosureStockBasedCompensationStockOptionsDetails STOCK-BASED COMPENSATION - Stock options (Details) Details 105 false false R106.htm 41804 - Disclosure - STOCK-BASED COMPENSATION - Restricted stock (Details) Sheet http://www.harborone.com/role/DisclosureStockBasedCompensationRestrictedStockDetails STOCK-BASED COMPENSATION - Restricted stock (Details) Details 106 false false R107.htm 41901 - Disclosure - STOCKHOLDERS' EQUITY (Details) Sheet http://www.harborone.com/role/DisclosureStockholdersEquityDetails STOCKHOLDERS' EQUITY (Details) Details http://www.harborone.com/role/DisclosureStockholdersEquity 107 false false R108.htm 42001 - Disclosure - COMPREHENSIVE INCOME (LOSS) (Details) Sheet http://www.harborone.com/role/DisclosureComprehensiveIncomeLossDetails COMPREHENSIVE INCOME (LOSS) (Details) Details http://www.harborone.com/role/DisclosureComprehensiveIncomeLossTables 108 false false R109.htm 42002 - Disclosure - COMPREHENSIVE INCOME (LOSS) - Changes in AOCI (Details) Sheet http://www.harborone.com/role/DisclosureComprehensiveIncomeLossChangesInAociDetails COMPREHENSIVE INCOME (LOSS) - Changes in AOCI (Details) Details http://www.harborone.com/role/DisclosureComprehensiveIncomeLossTables 109 false false R110.htm 42101 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES - Derivatives (Details) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesDerivativesDetails FAIR VALUE OF ASSETS AND LIABILITIES - Derivatives (Details) Details 110 false false R111.htm 42102 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES - Recurring basis (Details) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesRecurringBasisDetails FAIR VALUE OF ASSETS AND LIABILITIES - Recurring basis (Details) Details 111 false false R112.htm 42103 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES - Level 3 (Details) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesLevel3Details FAIR VALUE OF ASSETS AND LIABILITIES - Level 3 (Details) Details 112 false false R113.htm 42104 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES - Impaired Loans (Details) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesImpairedLoansDetails FAIR VALUE OF ASSETS AND LIABILITIES - Impaired Loans (Details) Details 113 false false R114.htm 42105 - Disclosure - FAIR VALUE OF ASSETS AND LIABILITIES - Balance Sheet Grouping (Details) Sheet http://www.harborone.com/role/DisclosureFairValueOfAssetsAndLiabilitiesBalanceSheetGroupingDetails FAIR VALUE OF ASSETS AND LIABILITIES - Balance Sheet Grouping (Details) Details 114 false false R115.htm 42201 - Disclosure - EARNINGS PER SHARE ("EPS") (Details) Sheet http://www.harborone.com/role/DisclosureEarningsPerShareepsDetails EARNINGS PER SHARE ("EPS") (Details) Details http://www.harborone.com/role/DisclosureEarningsPerShareTables 115 false false R116.htm 42301 - Disclosure - SEGMENT REPORTING (Details) Sheet http://www.harborone.com/role/DisclosureSegmentReportingDetails SEGMENT REPORTING (Details) Details http://www.harborone.com/role/DisclosureSegmentReportingTables 116 false false R117.htm 42401 - Disclosure - CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Balance Sheet (Details) Sheet http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompanyBalanceSheetDetails CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Balance Sheet (Details) Details 117 false false R118.htm 42402 - Disclosure - CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Statement Of Net Income (Details) Sheet http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompanyStatementOfNetIncomeDetails CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Statement Of Net Income (Details) Details 118 false false R119.htm 42403 - Disclosure - CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Statement of Cash Flows (Details) Sheet http://www.harborone.com/role/DisclosureCondensedFinancialStatementsOfParentCompanyStatementOfCashFlowsDetails CONDENSED FINANCIAL STATEMENTS OF PARENT COMPANY - Statement of Cash Flows (Details) Details 119 false false R120.htm 42501 - Disclosure - SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) Sheet http://www.harborone.com/role/DisclosureSelectedQuarterlyFinancialDataUnauditedDetails SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) Details http://www.harborone.com/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables 120 false false R121.htm 42601 - Disclosure - SUBSEQUENT EVENT (Details) Sheet http://www.harborone.com/role/DisclosureSubsequentEventDetails SUBSEQUENT EVENT (Details) Details http://www.harborone.com/role/DisclosureSubsequentEvent 121 false false All Reports Book All Reports hone-20191231.xml hone-20191231.xsd hone-20191231_cal.xml hone-20191231_def.xml hone-20191231_lab.xml hone-20191231_pre.xml http://fasb.org/us-gaap/2019-01-31 http://xbrl.sec.gov/dei/2019-01-31 http://fasb.org/srt/2019-01-31 true true