Represents the number of joint ventures in which entity has controlling interest.

No definition available.

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Period Type:	duration

- Definition

Represent the percentage of the company's rental income accounted by the tenant.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	num:percentItemType
Balance Type:	na
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- Definition

The carrying amount of investments in consolidated real estate and other joint ventures not separately presented. This includes direct and indirect investments.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51719941&loc=d3e2921-110230

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The book value of investments in real estate joint ventures including direct and indirect investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(d))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 19, 20, 22
-Article 5

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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- Details

Name:	us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=olp_ConsolidatedJointVentureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=olp_MCBRealEstateLLCAndItsAffiliatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=olp_PathmarkSupermarketMember
Namespace Prefix:
Data Type:	na
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Period Type:

Variable Interest Entities, Contingent Liabilities and Consolidated Joint Ventures - MCB Real Estate, LLC (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2016 USD ($) item	Jun. 30, 2015	Dec. 31, 2015 USD ($)
Variable Interest Entity, Nonconsolidated, Comparison of Carrying Amount of Assets and Liabilities to Maximum Loss Exposure
Carrying value of mortgage loans		$ 351,849,000		$ 331,055,000
Net book value of property		2,711,000
Impairment charge		$ 0
Consolidated JV
Variable Interest Entity, Nonconsolidated, Comparison of Carrying Amount of Assets and Liabilities to Maximum Loss Exposure
Number of joint ventures with controlling interest \| item		6
MCB Real Estate LLC And Its Affiliates \| Consolidated JV
Variable Interest Entity, Nonconsolidated, Comparison of Carrying Amount of Assets and Liabilities to Maximum Loss Exposure
Number of joint ventures with controlling interest \| item		5
Investment in consolidated joint ventures		$ 10,358,000
MCB Real Estate LLC And Its Affiliates \| Consolidated JV \| Pathmark supermarket in Philadelphia, Pennsylvania
Variable Interest Entity, Nonconsolidated, Comparison of Carrying Amount of Assets and Liabilities to Maximum Loss Exposure
Investment in consolidated joint ventures		2,895,000
Percentage of rental income	1.20%		1.20%
Carrying value of mortgage loans		4,455,000
Net book value of property		7,246,000
Impairment charge		$ 0