Cash outflow for the acquisition of land for the Company's ground leases

No definition available.

Name:	olp_ContractPurchasePriceOfLand
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the number of apartment units in the complex purchased during the period.

+ References

No definition available.

+ Details

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

- Definition

It Represents the number of VIEs of the company through ground leases.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the amount of mortgage balance related to third parties.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage.

+ References

No definition available.

+ Details

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

- Definition

The total amount of revenue recognized for the period from operating leases, including minimum lease revenue, contingent revenue, percentage revenue and sublease revenue.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902458&loc=d3e39896-112707

+ Details

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Data Type:	na
Balance Type:
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Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_LandMember
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Variable Interest Entities, Contingent Liabilities and Consolidated Joint Ventures - Ground Leases (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entities
Number of VIEs through ground leases \| item	2		2
Cash Reserves - ground lease	$ 379,000		$ 379,000		$ 443,000
Land, Lakemoor, Illinois
Variable Interest Entities
Revenue from the ground lease	214,000	$ 282,000	800,000	$ 862,000
The Briarbrook Village Apartments, Wheaton, Illinois and Vue Apartments, Beachwood, Ohio
Variable Interest Entities
Land Contract Purchase Price			$ 24,426,000
Units in Apartment Complex \| item			690
Owner/ Operator Mortgage from Third Party			$ 106,855,000
Carrying Amount and Maximum Exposure to Loss	24,437,000		24,437,000
The Briarbrook Village Apartments Wheaton, Illinois
Variable Interest Entities
Land Contract Purchase Price			$ 10,530,000
Units in Apartment Complex \| item			342
Owner/ Operator Mortgage from Third Party			$ 39,411,000
The Briarbrook Village Apartments Wheaton, Illinois \| Land
Variable Interest Entities
Carrying Amount and Maximum Exposure to Loss	10,536,000		10,536,000
The Vue Apartments, Beachwood, Ohio
Variable Interest Entities
Land Contract Purchase Price			$ 13,896,000
Units in Apartment Complex \| item			348
Owner/ Operator Mortgage from Third Party			$ 67,444,000
The Vue Apartments, Beachwood, Ohio \| Land
Variable Interest Entities
Carrying Amount and Maximum Exposure to Loss	13,901,000		13,901,000
The Briarbrook Village Apartment, Wheaton, Illinois; and Vue Apartment, Beachwood, Ohio
Variable Interest Entities
Revenue from the ground lease	$ 925,000	$ 954,000	$ 2,872,000	$ 2,758,000