Schedule III - Consolidated Real Estate and Accumulated Depreciation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014 property	Dec. 31, 2013	Dec. 31, 2012
Investment in real estate:
Balance, beginning of year	$ 574,424us-gaap_RealEstateGrossAtCarryingValue	$ 473,341us-gaap_RealEstateGrossAtCarryingValue	$ 430,337us-gaap_RealEstateGrossAtCarryingValue
Addition: Land, buildings and improvements	57,584us-gaap_RealEstateOtherAcquisitions	101,145us-gaap_RealEstateOtherAcquisitions	43,004us-gaap_RealEstateOtherAcquisitions
Deduction: Properties sold	(38,247)us-gaap_RealEstateCostOfRealEstateSold
Deduction: Impairment loss/charge	(1,093)	(62)
Balance, end of year	592,668us-gaap_RealEstateGrossAtCarryingValue	574,424us-gaap_RealEstateGrossAtCarryingValue	473,341us-gaap_RealEstateGrossAtCarryingValue
Accumulated depreciation:
Balance, beginning of year	73,060	62,816	54,214
Addition: Depreciation	12,064	10,244	8,602
Deduction: Accumulated depreciation related to properties sold and property contributed to joint venture	(7,481)
Balance, end of year	77,643	73,060	62,816
Other disclosure
Number of properties held-for-sale	2olp_NumberOfPropertiesHeldForSale
Amount by which aggregate cost of the properties is lower for federal income tax purposes	$ 1,808

Represents the number of properties held-for-sale during the period.

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- Definition

Represents the amount by which aggregate cost of the properties is lower or higher for federal income tax purposes.

+ References

No definition available.

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- Definition

Amount of accumulated depreciation pertaining to real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.FN4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

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- Definition

Amount of decrease to accumulated depreciation for real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

+ Details

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- Definition

Amount of real estate investments sold for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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- Definition

Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

+ Details

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- Definition

Amount of increase in real estate investments from acquisitions other than through foreclosure for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

+ Details

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- Definition

Amount of depreciation expense of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

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- Definition

Amount of write-down or reserve recognized in the income statement on real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.FN5)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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