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Balance Sheet Details - Schedule of Other Assets (Parenthetical) (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Other Assets [Abstract] |  |
Allowance for convertible notes receivable | $ 2,000 | $ 2,000

X
- Definition Allowance for convertible note receivable noncurrent. + References No definition available. + Details
Name: | onto_AllowanceForConvertibleNoteReceivableNoncurrent
Namespace Prefix: | onto_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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