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Variable Interest Entities, Contingent Liabilities and Consolidated Joint Ventures - Consolidated Joint Ventures (Details) - Consolidated JV	6 Months Ended
	Jun. 30, 2018 item
Variable Interest Entities
Number of joint ventures with controlling interest	5
Minimum
Variable Interest Entities
Ownership interest in consolidated joint venture of the company (as a percent)	90.00%
Maximum
Variable Interest Entities
Ownership interest in consolidated joint venture of the company (as a percent)	95.00%

X

- Definition

Represents the number of joint ventures in which entity has controlling interest.

+ References

No definition available.

+ Details

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

X

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- 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

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

X

- Details

Name:	us-gaap_InvestmentsInAndAdvancesToAffiliatesCategorizationAxis=olp_ConsolidatedJointVentureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: