Amount of difference between the carrying amount and the sales price of real estate properties sold.

No definition available.

Name:	olp_GainLossOnDispositionOfRealEstateInUnconsolidatedJointVenture
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The number of acres of land retained by unconsolidated joint venture.

+ References

No definition available.

+ Details

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Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of properties owned and operated by each unconsolidated joint venture.

+ References

No definition available.

+ Details

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of unconsolidated joint ventures of the entity that are accounted for under the equity method of accounting.

+ References

No definition available.

+ Details

Name:	olp_NumberOfUnconsolidatedJointVentures
Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of share of gain on sale of property in unconsolidated joint venture.

+ References

No definition available.

+ Details

Name:	olp_PercentageOfShareOfGainOnSaleOfPropertyInUnconsolidatedJointVenture
Namespace Prefix:	olp_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from the sale of buildings and land.

+ References

No definition available.

+ Details

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

- Definition

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

+ 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(12))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(10))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66022249&loc=d3e33749-111570

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(13)(f))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

The carrying amount of investments in unconsolidated real estate and other joint ventures not separately presented. This includes 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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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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- Details

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Investment in Unconsolidated Joint Ventures (Details)		3 Months Ended		12 Months Ended
Investment in Unconsolidated Joint Ventures (Details)	Apr. 05, 2018 USD ($) item	Mar. 31, 2018 USD ($) property item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) property item
Investment in Unconsolidated Joint Ventures
Number of unconsolidated joint ventures \| item		5		5
Number of properties owned and operated by each unconsolidated joint venture \| property		1		1
Investment in unconsolidated joint ventures		$ 10,921,000		$ 10,723,000
Equity in earnings of unconsolidated joint ventures		195,000	$ 245,000
Share of gain (as a percent)	50.00%
Gain on sale of real estate	$ 70,000
Acres of land retained \| item	5
Georgia
Investment in Unconsolidated Joint Ventures
Proceeds from the sale of a building and a portion of the land	$ 2,600,000
MCB Real Estate LLC And Its Affiliates
Investment in Unconsolidated Joint Ventures
Investment in unconsolidated joint ventures		$ 8,324,000