Number of tranches for the performance based awards.

No definition available.

Name:	ovas_NumberOfTranches
Namespace Prefix:	ovas_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the minimum percentage required to meet a performance criteria.

+ References

No definition available.

+ Details

Name:	ovas_PerformanceCriteriaPercentage
Namespace Prefix:	ovas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the annual increase to the maximum number of shares (or other type of equity) approved (usually by shareholders and board of directors) for awards under the equity-based compensation plan, as a percentage of the number of shares of the entity's common stock then outstanding.

+ References

No definition available.

+ Details

Name:	ovas_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncreaseInNumberOfSharesAuthorizedAsPercentageOfSharesOutstanding
Namespace Prefix:	ovas_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

An excess of the fair value of the modified award over the fair value of the award immediately before the modification.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)(iii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardPlanModificationIncrementalCompensationCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of vesting of share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_TitleOfIndividualAxis=ovas_NonEmployeeStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=ovas_FounderMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=ovas_StockOptions1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_StockCompensationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_PresidentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=us-gaap_ChiefExecutiveOfficerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=ovas_December2014AwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=ovas_VestingBasedOnServiceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=ovas_VestingBasedOnPerformanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TitleOfIndividualAxis=ovas_SeniorExecutivesandNonEmployeeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=ovas_StockIncentivePlan2012Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=ovas_EquityIncentivePlan2011Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation - Narrative (Details)				1 Months Ended		3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
Stock-Based Compensation - Narrative (Details)	Dec. 03, 2015 $ / shares shares	May 29, 2015 $ / shares	Dec. 09, 2014 tranche $ / shares shares	Dec. 31, 2012	Mar. 31, 2012 shares	Jun. 30, 2018 $ / shares shares	Jun. 30, 2017 $ / shares shares	Jun. 30, 2017 $ / shares shares	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Jun. 21, 2017 USD ($)	Dec. 05, 2012 tranche
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)						1,179,877	2,183,106	4,015,356		4,386,856
Weighted average exercise price (in dollars per share) \| $ / shares						$ 0.96	$ 1.46	$ 1.51		$ 1.51
Weighted average period for recognition of compensation cost related to unvested stock awards										2 years 7 months 17 days
Total unrecognized compensation cost, net of estimated forfeitures, related to non-vested stock options and restricted stock \| $										$ 4,200,000
RSU's granted (in dollars per share) \| $ / shares						$ 0.70	$ 1.11	$ 1.15
Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)							150,000	150,000
Weighted average exercise price (in dollars per share) \| $ / shares							$ 1.60	$ 1.60
Founders'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)												329,021
Allocated share-based compensation expense \| $										0	$ 0	$ 3,400,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense \| $										$ 6,600,000	8,700,000	14,100,000
Total intrinsic value \| $											$ 300,000	$ 6,700,000
Options granted (in shares)										4,236,856	2,570,600	1,744,600
Weighted average exercise price (in dollars per share) \| $ / shares										$ 1.51	$ 6.80	$ 33.13
Weighted average grant date fair value of options granted (in dollars per share) \| $ / shares										$ 1.14	$ 4.84	$ 20.87
Stock options \| Non-employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense \| $										$ 300,000	$ 100,000	$ 1,100,000
Options granted (in shares)										150,000	10,000	50,500
Weighted average exercise price (in dollars per share) \| $ / shares										$ 1.60	$ 6.96	$ 36.25
Stock Compensation Plan [Member] \| President [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average period for recognition of compensation cost related to unvested stock awards										2 years
Stock-based compensation expense \| $									$ 100,000
Total unrecognized compensation cost, net of estimated forfeitures, related to non-vested stock options and restricted stock \| $													$ 2,700,000
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)										0
Options granted (in shares)										0
RSU's granted (in dollars per share) \| $ / shares										$ 0.00
Total fair value of RSUs that vested \| $										$ 0	$ 100,000	$ 500,000
Restricted stock units \| Tranche one
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	75,000
Vesting percentage	25.00%
Restricted stock units \| Tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	10,000
Restricted stock units \| Chief Executive Officer \| December 2014 Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense \| $												700,000
Weighted average exercise price (in dollars per share) \| $ / shares		$ 43.47
Performance criteria			150.00%
Restricted stock units \| Chief Executive Officer \| Service-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period (in years)			24 months	24 months
Restricted stock units \| Chief Executive Officer \| Service-based restricted stock units \| December 2014 Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)			15,450
Allocated share-based compensation expense \| $												500,000
Options granted (in shares)			30,902
RSU's granted (in dollars per share) \| $ / shares			$ 32.36
Restricted stock units \| Chief Executive Officer \| Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of tranches \| tranche			2											2
Vesting period (in years)			1 year	1 year
Restricted stock units \| Chief Executive Officer \| Performance-based restricted stock units \| December 2014 Award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense \| $												$ 200,000
Restricted stock units \| Senior executives and a non-employee consultant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense \| $											$ 300,000
Options granted (in shares)	85,000
RSU's granted (in dollars per share) \| $ / shares	$ 9.81
Restricted stock units \| Senior executives and a non-employee consultant \| Tranche two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)											16,250
Options granted (in shares)											85,000
Vesting period (in years)	12 months
Forfeitures (in shares)											68,750
2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)					1,453,253
2012 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase to the lowest number of shares of common stock (in shares)					975,000
Annual increase as a percentage of the number of shares of common stock outstanding					4.00%
2011 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future issuance (in shares)					679,622