Aggregate fees paid to other partners of the venture.

No definition available.

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

- Definition

Reduction of equity in earnings of unconsolidated joint ventures on the consolidated statement of income.

+ References

No definition available.

+ Details

Name:	olp_DecreaseInEquityInEarningsJointVentureTransaction
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents information pertaining to reimbursement of insurance cost.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	olp_RealEstatePropertyManagementCosts
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of additional payment to related party for the share of direct office expenses.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionAdditionalPaymentForShareOfDirectOfficeExpenses
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount paid to related parties under related party transaction.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionAmountPaid
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount to be paid under the agreement with the related party.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionFees
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of property management costs allocated to real estate expenses.

+ References

No definition available.

+ Details

Name:	olp_RelatedPartyTransactionPropertyManagementCostAllocatedToRealEstateExpenses
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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=96867065&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=96867065&loc=d3e5070-113901

+ Details

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

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyManagementFeePercentFee
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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=olp_ExecutiveOfficersAndOthersMember
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_RelatedPartyTransactionsByRelatedPartyAxis=us-gaap_CorporateJointVentureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_GeneralAndAdministrativeExpenseMember
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Data Type:	na
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- Details

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

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

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

Name:	us-gaap_MajorPropertyClassAxis=olp_OperatingLeasePropertyMember
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Related Party Transactions (Details) - USD ($)	3 Months Ended		6 Months Ended
Related Party Transactions (Details) - USD ($)	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Related Party Transaction
Share based compensation charged to operations	$ 856,000	$ 915,000	$ 1,682,000	$ 1,657,000
Majestic
Related Party Transaction
Fees under compensation and services agreement	689,000	664,000	1,367,000	1,329,000
Property management costs allocated to real estate expenses	309,000	284,000	608,000	570,000
Additional payment for the entity's share of all direct office expenses	54,000	54,000	108,000	108,000
Executive officers and others
Related Party Transaction
Share based compensation charged to operations	432,000	386,000	849,000	768,000
Joint venture partners
Related Party Transaction
Real estate property management costs	22,000	33,000	65,000	82,000
Corporate joint venture
Related Party Transaction
Aggregate fees paid to other partners	45,000	42,000	96,000	87,000
Decrease in equity earnings, joint venture transaction	23,000	21,000	48,000	44,000
Chairman \| General and administrative expense
Related Party Transaction
Fee paid	69,000	69,000
Vice Chairman \| General and administrative expense
Related Party Transaction
Fee paid	27,500	27,500
Gould Investors L.P.
Related Party Transaction
Insurance Reimbursement	$ 206,000	$ 174,000	$ 404,000	$ 347,000
Net lease tenants \| Majestic
Related Party Transaction
Property management fee (as a percent)			1.50%
Operating lease tenants \| Majestic
Related Party Transaction
Property management fee (as a percent)			2.00%