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v3.20.1

Share Repurchase Authorization - Textual (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 28, 2020 | Mar. 14, 2019
Shares Repurchase Authorization [Line Items] |  |
Amount available for future repurchase | $ 46,385,000 |
Stock repurchase, authorization termination date | Oct. 25, 2019 |
Maximum [Member] |  |
Shares Repurchase Authorization [Line Items] |  |
Stock repurchase, authorized amount |  | $ 80,000,000

X
- Definition Shares repurchase authorization. + References No definition available. + Details
Name: | onto_SharesRepurchaseAuthorizationLineItems
Namespace Prefix: | onto_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramAuthorizedAmount1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Expiration date for the purchase of an entity's own shares under a stock repurchase plan, in the CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramExpirationDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount remaining of a stock repurchase plan authorized. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |