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SIGNIFICANT ACCOUNTING POLICIES - Principles of Consolidation (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Investment in Joint Ventures and Variable Interest Entities
Number of Unconsolidated Joint Venture VIEs | item	0
Recourse debt of joint venture	$ 0
Impairment charge relating to investments in unconsolidated joint ventures	0
Revenue Recognition
Additional rental income for the reimbursement of expenses	$ 7,672,000	$ 6,424,000	$ 3,852,000
Purchase Accounting for Acquisition of Real Estate
Estimated useful lives of intangible assets, minimum	1 year
Estimated useful lives of building and intangible assets, maximum	38 years
Escrow, Deposits and Other Assets and Receivables
Real estate taxes, insurance and other escrows	$ 460,000	$ 387,000

X

- Definition

Minimum useful life of finite-lived intangible assets or liabilities used in the normal conduct of business and not intended for resale.

+ References

No definition available.

+ Details

Name:	olp_IntangibleAssetsLiabilitiesUsefulLifeMinimum
Namespace Prefix:	olp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of debt for which the creditor has general recourse to the joint venture debtor.

+ References

No definition available.

+ Details

Name:	olp_JointVentureRecourseDebt
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Maximum useful life of finite-lived intangible assets and long lived, physical assets used in the normal conduct of business and not intended for resale.

+ References

No definition available.

+ Details

Name:	olp_PropertyPlantAndEquipmentAndIntangibleAssetsLiabilitiesUsefulLifeMaximum
Namespace Prefix:	olp_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Represents the amounts related to Real estate taxes, insurance and other escrows related to real estate in escrow, deposits and other assets and receivables.

+ References

No definition available.

+ Details

Name:	olp_ReTaxesInsuranceAndOtherEscrows
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Number of unconsolidated joint venture investments that are classified as a Variable Interest Entity (VIE).

+ References

No definition available.

+ Details

Name:	olp_UnconsolidatedJointVentureVariableInterestEntityNumberOfEntities
Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

+ Details

Name:	us-gaap_ImpairmentOfInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_RealEstateInvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_RestrictedCashAndInvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_RevenueRecognitionAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

In accordance with the provisions of their lease agreement, this element represents allowable charges due a landlord from its tenant. In retail store and office building leases, for example, tenant reimbursements may cover items such as taxes, utilities, and common area expenses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_TenantReimbursements
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityNotPrimaryBeneficiaryDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration