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LEASES - As Lessee - Ground Lease and Office Lease (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) item	Jan. 01, 2019 USD ($)
Lessee
Operating lease liability	$ 4,803,000
Operating lease liability, Statement of Financial Position	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent
Five-Year Lease
Lessee
Operating lease, number of renewal options | item	4
Operating lease, renewal term	5 years
Seven-Month Lease
Lessee
Operating lease, number of renewal options | item	1
Operating lease, renewal term	7 months
Real Estate in Greensboro, NC
Lessee
Operating lease, option to extend	true
Operating lease, discount rate (as a percent)	4.75%
Operating lease, remaining lease term	10 years 2 months 12 days
Operating lease expense	$ 525,000
Real Estate in Greensboro, NC | Five-Year Lease
Lessee
Operating lease, number of renewal options | item	4
Operating lease, renewal term	5 years
Real Estate in Greensboro, NC | Seven-Month Lease
Lessee
Operating lease, number of renewal options | item	1
Operating lease, renewal term	7 months
Real Estate in Greensboro, NC | ASU 2016-02 | Adjustment
Lessee
Operating lease liability		$ 4,381,000
Operating lease liability, Statement of Financial Position		us-gaap:AccruedLiabilitiesCurrentAndNoncurrent
Right-of-use asset		$ 4,381,000
Operating lease, right-of-use asset, Statement of Financial Position		olp:EscrowDepositsAndOtherAssetsAndReceivables
Corporate office lease in Great Neck
Lessee
Operating lease, option to extend	true
Operating lease, renewal term	5 years
Operating lease liability	$ 646,000
Operating lease liability, Statement of Financial Position	us-gaap:AccruedLiabilitiesCurrentAndNoncurrent
Right-of-use asset	$ 646,000
Operating lease, right-of-use asset, Statement of Financial Position	olp:EscrowDepositsAndOtherAssetsAndReceivables
Operating lease, discount rate (as a percent)	3.81%
Operating lease, remaining lease term	17 years
Operating lease expense	$ 54,000

X

- Definition

The number of renewal options available to the Company, as defined in the lease agreement.

+ References

No definition available.

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+ References

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- Definition

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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

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- Definition

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+ References

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-Publisher FASB
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- Definition

Term of lessee's operating lease renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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-Publisher FASB
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-SubTopic 20
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- Definition

Amount of operating lease expense. Excludes sublease income.

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- Definition

Present value of lessee's discounted obligation for lease payments from operating lease.

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- Definition

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- Definition

Amount of lessee's right to use underlying asset under operating lease.

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- Definition

Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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