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Summary Accounting Policies - Investment in Joint Ventures and Variable Interest Entities (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) item	Mar. 31, 2018 USD ($)
Investment in Joint Ventures and Variable Interest Entities
Number of Unconsolidated Joint Venture VIEs | item	0
Recourse debt of joint venture	$ 0
Impairment charge relating to investments in unconsolidated joint ventures	$ 0	$ 0

X

- Definition

Amount of debt for which the creditor has general recourse to the joint venture debtor.

+ References

No definition available.

+ Details

Name:	olp_JointVentureRecourseDebt
Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Number of unconsolidated joint venture investments that are classified as a Variable Interest Entity (VIE).

+ References

No definition available.

+ Details

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

X

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

+ Details

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

X

- References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityNotPrimaryBeneficiaryDisclosuresAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration