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Organization and Operations of Company - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Jun. 22, 2010 OAS Holding Company LLC [Member]	Jun. 22, 2010 IPO [Member]
Organization And Operations Of The Company [Line Items]
Sale of common stock		93,432,712	92,483,393	17,930,000	30,370,000
Sale of common stock, price per share				$ 14.00	$ 14.00
Underwriting discounts and commissions					$ 25,500,000
Proceeds from issuance of initial public offering	$ 399,669,000			$ 236,000,000

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Fees and commissions paid for the securities underwriting activities related to the initial public offering.

+ References

No definition available.

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.29)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 30

-Article 5

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The cash inflow associated with the amount received from entity's first offering of stock to the public.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 19

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 18

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 14

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3255-108585

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Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

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-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section H

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