FAIR VALUE - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Transfers between Level I and Level II	$ 1,066.8
Level III [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value of Debt Obligations	651.1	652.9
Average borrowing rates	3.10%	3.10%
Percentage increase in average borrowing rate assumption that would lower fair value of debt obligation	10.00%
Fair value of debt obligation, decrease in value due to increase in average borrowing rate	644.3
Percentage decrease in average borrowing rate assumption that would increase fair value of debt obligation	10.00%
Fair value of debt obligation, increase in value due to decrease in average borrowing rate	659.7
Level II [Member] \| Swaps (net)-corporate debt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Accounts payable, other accrued expenses and other liabilities	$ 7.4	$ 7.9

Average borrowing rates used for level three valuation of debt obligations.

No definition available.

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Period Type:	instant

- Definition

Fair value of debt obligation, decrease in value due to increase in average borrowing rate.

+ References

No definition available.

+ Details

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Namespace Prefix:	oak_
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Period Type:	instant

- Definition

Fair value of debt obligation, increase in value due to decrease in average borrowing rate.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

Percentage decrease in average borrowing rate assumption that would increase fair value of debt obligation.

+ References

No definition available.

+ Details

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Namespace Prefix:	oak_
Data Type:	num:percentItemType
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Period Type:	instant

- Definition

Percentage increase in average borrowing rate assumption that would lower fair value of debt obligation.

+ References

No definition available.

+ Details

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Namespace Prefix:	oak_
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Balance Type:	na
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13476-108611

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 15

-Paragraph 83

-URI http://asc.fasb.org/extlink&oid=6913464&loc=d3e34841-113949

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 10

-Subparagraph i

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 6

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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- Definition

Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bb)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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Period Type:	duration