The rate of forfeitures assumptions that is used in calculating the fair values of a share-based compensation award.

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- Definition

Derivative, Number of Counterparties

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- Definition

The notional volumes in commodity derivative contracts as a percentage of current production.

+ References

No definition available.

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- Definition

Spot Price Decrease, Crude Oil, Percentage

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Spot Price Decrease, Natural Gas, Percentage

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- Definition

Represents the expense recognized during the period arising from stock-based equity awards granted to employees and directors that has not yet been realized for excess tax benefits.

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- Definition

The amount of exploratory well costs that have been capitalized for a period of greater than one year after the completion of drilling.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph b

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- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 7A

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953401-111524

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- Definition

Amount of gain (loss) on sale or disposal of oil and gas property or timber property.

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- Definition

The expense recorded to reduce the value of oil and gas assets consisting of proved properties and unproved properties as the estimate of future successful production from these properties is reduced.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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-Name Accounting Standards Codification

-Topic 932

-SubTopic 360

-Section 35

-Paragraph 11

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-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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- Definition

Amount of interest capitalized during the period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Allowance for Funds Used during Construction

-URI http://asc.fasb.org/extlink&oid=6504829

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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- Definition

The cash inflow to dispose of long-lived, physical assets and mineral interests in oil and gas properties used for normal oil and gas operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 12

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585

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- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended	9 Months Ended
	Dec. 31, 2014 counterparty	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Feb. 17, 2015
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Proceeds from sale of property	$ 324,852,000
Gain on sale of properties	186,999,000us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty	0us-gaap_GainLossOnSaleOfProperty
Impairment of oil and gas properties	47,238,000us-gaap_ImpairmentOfOilAndGasProperties	1,168,000us-gaap_ImpairmentOfOilAndGasProperties	3,581,000us-gaap_ImpairmentOfOilAndGasProperties
Capitalized interest costs	8,800,000us-gaap_InterestCostsCapitalized	4,600,000us-gaap_InterestCostsCapitalized	3,300,000us-gaap_InterestCostsCapitalized
Capitalized Exploratory Well Costs that Have Been Capitalized for Period Greater than One Year	0
Derivative, number of counterparties	7oas_DerivativeNumberofCounterparties
Past-due receivables from counterparties	0
Restricted stock awards vesting period	3 years
1 to 12 months [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Contractual commodity derivative volume, percent	110.00%
13 to 24 months [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Contractual commodity derivative volume, percent	100.00%
25 to 36 months [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Contractual commodity derivative volume, percent	75.00%
37 to 60 months [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Contractual commodity derivative volume, percent	50.00%
Minimum [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Annual Forfeiture Rate	0.00%
Maximum [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Annual Forfeiture Rate	12.70%
Non-Operated OIl and Natural Gas Properties [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Proceeds from sale of property				324,900,000
Gain on sale of properties				187,000,000
Building [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Estimated life of assets	20 years
Proved oil and gas properties [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Impairment of oil and gas properties	40,000,000	0	0
Unproved Oil And Gas Properties [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Impairment of oil and gas properties	7,300,000	1,200,000	3,600,000
Salt Water Disposal Facility [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Estimated life of assets	30 years
Furniture and equipment [Member] \| Minimum [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Estimated life of assets	2 years
Furniture and equipment [Member] \| Maximum [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Estimated life of assets	7 years
Domestic Tax Authority [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Stock-based compensation related to unrealized excess tax benefits	9,500,000
State and Local Jurisdiction [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Stock-based compensation related to unrealized excess tax benefits	$ 7,400,000
Subsequent Event [Member]
Interest Cost Capitalized And Depreciation And Amortization Expenses For Property, Plant and Equipment [Line Items]
Spot Price Decrease, Crude Oil, Percentage					50.00%
Spot Price Decrease, Natural Gas, Percentage					40.00%