The amount of decrease in income from entity's investee (such as unconsolidated subsidiaries and joint ventures).

No definition available.

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

- Definition

Amount paid to managing member or general partner for management of the day-to-day business functions.

+ References

No definition available.

+ Details

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

- Definition

Represents information pertaining to reimbursement of insurance cost.

+ References

No definition available.

+ Details

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

- Definition

Represents the aggregate costs related to management and acquisition of properties during the reporting period.

+ References

No definition available.

+ Details

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

- Definition

Represents the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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

- Definition

Represents management fees included in the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Represents overhead expenses included in the quarterly amount to be paid under the agreement with the related party for the current fiscal year.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

A contract to provide coverage or protection in exchange for a payment or "premium". Examples of insurance protection include liability and property insurance. The entity paying the premiums for the protection will have insurance expense and possibly an asset, Prepaid Insurance (if the premiums are paid in advance).

+ 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 210.5-03.6)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

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

- 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

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

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_MajesticPropertyManagementCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_BoardOfDirectorsChairmanMember
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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_BoardOfDirectorsViceChairmanMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_ExecutiveOfficersAndOthersMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=olp_GouldInvestorsLPMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_CoVenturerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorPropertyClassAxis=olp_NetLeasePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorPropertyClassAxis=olp_OperatingLeasePropertyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=olp_UnconsolidatedJointVenturesMember
Namespace Prefix:
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Related Party Transactions (Details) - USD ($)		3 Months Ended		9 Months Ended
Related Party Transactions (Details) - USD ($)	Jan. 01, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Majestic
Related Party Transaction
Quarterly fees under compensation and services agreement		$ 678,000	$ 634,000	$ 2,002,000	$ 1,901,000
Management fees under compensation and services agreement		267,000	223,000	770,000	669,000
Overhead expenses under compensation and services agreement		49,000	49,000	147,000	147,000
Chairman
Related Party Transaction
Quarterly fees under compensation and services agreement				65,625	65,625
Vice Chairman
Related Party Transaction
Quarterly fees under compensation and services agreement				26,250	26,250
Executive officers and others
Related Party Transaction
Share based compensation charged to operations		399,000	307,000	1,125,000	930,000
Gould Investors L.P.
Related Party Transaction
Insurance Reimbursement		699,000	513,000	699,000	513,000
Real estate insurance expense		169,000	121,000	371,000	235,000
Joint venture partners
Related Party Transaction
Real estate management and acquisition costs		35,000	29,000	123,000	167,000
Net lease tenants
Related Party Transaction
Property management fee (as a percent)	1.50%
Operating lease tenants
Related Party Transaction
Property management fee (as a percent)	2.00%
Unconsolidated JV \| Joint venture partners
Related Party Transaction
Management fees		55,000	35,000	127,000	361,000
Reduction of equity in earnings of unconsolidated joint ventures		$ 27,000	$ 18,000	$ 63,000	$ 181,000