INCOME TAXES - Carryforwards (Details) - USD ($)		1 Months Ended	12 Months Ended
	Aug. 14, 2019	Jun. 30, 2016	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Tax Act income tax provision amount			$ 243,000
Net deferred tax asset balance			6,034,000	$ 843,000	$ 6,727,000	$ 610,000
Charitable contribution and other carryforwards			15,000		380,000
Shares issued	31,036,812
Temporary difference adjustment				320,000
Charitable contribution carryforward expected to be used			1,100,000
Request tax refunds				$ 3,200,000
Federal
Operating Loss Carryforwards [Line Items]
Charitable contribution and other carryforwards					1,100,000
Federal | Tax Year 2013
Operating Loss Carryforwards [Line Items]
Tax refund			603,000
Federal | Tax Year 2014
Operating Loss Carryforwards [Line Items]
Tax refund					826,000
Federal | Tax Year 2015
Operating Loss Carryforwards [Line Items]
Tax refund					$ 1,300,000
State | Tax Year 2013
Operating Loss Carryforwards [Line Items]
Tax refund			211,000
State | Tax Year 2014
Operating Loss Carryforwards [Line Items]
Tax refund			320,000
State | Tax Year 2015
Operating Loss Carryforwards [Line Items]
Tax refund			40,000
HarborOne Foundation
Operating Loss Carryforwards [Line Items]
Shares issued		385,450
Cash		$ 965,000
HarborOne Bank | Maximum
Operating Loss Carryforwards [Line Items]
Net operating loss that can be used per year			552,000
HarborOne Bank | New Hampshire | HarborOne Mortgage
Operating Loss Carryforwards [Line Items]
Net operating loss			2,200,000
Net deferred tax asset balance			$ 15,000