Balance Sheet Details - Schedule of Other Assets (Parenthetical) (Details) - USD ($) $ in Thousands	Mar. 28, 2020	Dec. 31, 2019
Other Assets [Abstract]
Allowance for convertible notes receivable	$ 2,000	$ 2,000

Allowance for convertible note receivable noncurrent.

No definition available.

Name:	onto_AllowanceForConvertibleNoteReceivableNoncurrent
Namespace Prefix:	onto_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant