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Segment Reporting - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2016 USD ($) Segment
Segment Reporting [Abstract] |
Number of reportable segments | Segment | 2
Intersegment sales | $ | $ 0

X
- Definition Intersegment sales. + References No definition available. + Details
Name: | cdna_IntersegmentSales
Namespace Prefix: | cdna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements. + References No definition available. + Details
Name: | us-gaap_NumberOfReportableSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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