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Fair value (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2013 | Jun. 30, 2013 Fair value measurements on a recurring basis Level 1 | Jun. 30, 2013 Fair value measurements on a recurring basis Level 2 | Jun. 30, 2013 Fair value measurements on a recurring basis Cash and money market funds Level 1 | Jun. 30, 2013 Fair value measurements on a recurring basis Corporate obligations (including commercial paper) Level 2 | Jun. 30, 2013 Fair value measurements on a recurring basis Short-term investments Level 3 | Dec. 31, 2012 Fair value measurements on a recurring basis Short-term investments Level 3
Assets: |  |  |  |  |  |  |  |
Total |  |  | $ 28,193 | $ 26,152 | $ 28,193 | $ 26,152 | $ 0 | $ 0
Transfers of assets from Level 1 to Level 2 | 0 | 0 |  |  |  |  |  |
Transfers of assets from Level 2 to Level 1 | 0 | 0 |  |  |  |  |  |
Transfers of liabilities from Level 1 to Level 2 | 0 | 0 |  |  |  |  |  |
Transfers of liabilities from Level 2 to Level 1 | $ 0 | $ 0 |  |  |  |  |  |

X
- Definition Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References No definition available. + Details
Name: | ovas_FairValueAssetsLevel1ToLevel2TransferAmount
Namespace Prefix: | ovas_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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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 No definition available. + Details
Name: | ovas_FairValueAssetsLevel2ToLevel1TransferAmount
Namespace Prefix: | ovas_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of transfers of liabilities measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2. + References No definition available. + Details
Name: | ovas_FairValueLiabilitiesLevel1ToLevel2TransferAmount
Namespace Prefix: | ovas_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount of transfers of liabilities measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1. + References No definition available. + Details
Name: | ovas_FairValueLiabilitiesLevel2ToLevel1TransferAmount
Namespace Prefix: | ovas_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. + 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 (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_AssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_AssetsFairValueDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration