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v3.19.3

Accounts Receivable - Textual (Details) - USD ($) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 28, 2019 | Sep. 29, 2018 | Sep. 28, 2019 | Sep. 29, 2018 | Dec. 29, 2018
Accounts Receivable (Additional Textual) [Abstract] |  |  |  |  |
Sold receivables amount | $ 4,400,000 | $ 12,800,000 | $ 18,700,000 | $ 47,500,000 |
Due from unrelated third parties | $ 0 |  | $ 0 |  | $ 0
Minimum [Member] |  |  |  |  |
Accounts Receivable (Textual) [Abstract] |  |  |  |  |
Administrative fees as well as interest percent |  |  | 0.61% |  |
Maximum [Member] |  |  |  |  |
Accounts Receivable (Textual) [Abstract] |  |  |  |  |
Administrative fees as well as interest percent |  |  | 1.68% |  |

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- Definition Accounts receivable. + References No definition available. + Details
Name: | onto_AccountsReceivableTextualAbstract
Namespace Prefix: | onto_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Administrative fees and interest percent. + References No definition available. + Details
Name: | onto_AdministrativeFeesAndInterestPercent
Namespace Prefix: | onto_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Due from unrelated third parties. + References No definition available. + Details
Name: | onto_DueFromUnrelatedThirdParties
Namespace Prefix: | onto_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Proceeds from accounts receivable. + References No definition available. + Details
Name: | onto_ProceedsFromAccountsReceivable
Namespace Prefix: | onto_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_AccountsReceivableAdditionalDisclosuresAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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

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