Related Party Transactions (Details) - USD ($)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction
Share based compensation charged to operations	$ 826,000	$ 742,200
Majestic
Related Party Transaction
Fees under compensation and services agreement	678,000	665,000
Property management costs allocated to real estate expenses	299,000	285,000
Additional payment for the entity's share of all direct office expenses	54,000	54,000
Executive officers and others
Related Party Transaction
Share based compensation charged to operations	417,000	381,000
Joint venture partners
Related Party Transaction
Real estate property management costs	43,000	49,000
Corporate joint venture
Related Party Transaction
Aggregate fees paid to other partners	51,000	46,000
Decrease in equity earnings, joint venture transaction	25,000	23,000
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	$ 201,000	$ 173,000
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%

Aggregate fees paid to other partners of the venture.

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Represents information pertaining to reimbursement of insurance cost.

+ References

No definition available.

+ Details

Name:	olp_RealEstateInsuranceReimbursement
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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- Definition

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

+ References

No definition available.

+ Details

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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

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Balance Type:	debit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- 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

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

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

The percentage charged for managing real estate properties.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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- 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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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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