v2.4.1.9

Warrants - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Sep. 30, 2014	Oct. 31, 2011
Class of Warrant or Right [Line Items]
Common stock price	$ 8.97us-gaap_SharePrice	$ 7.25us-gaap_SharePrice	$ 7.03us-gaap_SharePrice
Dividend yield	0.00%us-gaap_FairValueAssumptionsExpectedDividendRate
Warrants to purchase convertible preferred stock				82,190cdna_WarrantsToPurchaseConvertiblePreferredStock
Minimum [Member]
Class of Warrant or Right [Line Items]
Expected volatility	40.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_RangeAxis = us-gaap_MinimumMember
Risk-free interest rate	0.80%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member]
Class of Warrant or Right [Line Items]
Expected volatility	45.00%us-gaap_FairValueAssumptionsExpectedVolatilityRate / us-gaap_RangeAxis = us-gaap_MaximumMember
Risk-free interest rate	2.10%us-gaap_FairValueAssumptionsRiskFreeInterestRate / us-gaap_RangeAxis = us-gaap_MaximumMember

X

- Definition

Warrants To Purchase Convertible Preferred Stock

+ References

No definition available.

+ Details

Name:	cdna_WarrantsToPurchaseConvertiblePreferredStock
Namespace Prefix:	cdna_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_ClassOfWarrantOrRightLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsExpectedDividendRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsExpectedVolatilityRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Risk-free interest rate assumption used in valuing an instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssumptionsRiskFreeInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: