Related Party Transactions (Details) - USD ($)	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Related Party Transaction
Share based compensation charged to operations	$ 742,200	$ 665,700
Majestic
Related Party Transaction
Quarterly fees under compensation and services agreement	665,000	603,000
Property management costs allocated to real estate expenses annually	285,000	241,000
Additional payment for the entity's share of all direct office expenses	54,000	49,000
Executive officers and others
Related Party Transaction
Share based compensation charged to operations	381,000	344,000
Joint venture partners
Related Party Transaction
Real estate property management costs	49,000	51,000
Corporate joint venture
Related Party Transaction
Aggregate fees paid to other partners	46,000	35,000
Decrease in equity earnings, joint venture transaction	23,000	17,500
Chairman
Related Party Transaction
Fee paid	69,000	65,625
Vice Chairman
Related Party Transaction
Fee paid	27,500	26,250
Gould Investors L.P.
Related Party Transaction
Real estate insurance expense	$ 173,000	$ 101,000
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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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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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 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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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

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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 annually.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
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
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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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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

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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

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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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- Details

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