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Variable Interest Entities, Contingent Liabilities and Consolidated Joint Ventures - Consolidated Joint Ventures (Details) - Consolidated JV | 3 Months Ended
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Mar. 31, 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: |