For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=64851502&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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-Section 50
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Paragraph 21
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Cash and equivalents whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
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The amount of an asset, typically cash, provided to a counterparty to provide certain assurance of performance by the entity pursuant to the terms of a written or oral agreement, such as a lease.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Represents the number of major financial institutions, with which cash and cash equivalents are deposited by the entity.

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Summary of Significant Accounting Policies - Credit Risk, Restricted Cash, and Allowance for Doubtful Accounts (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) Institution	Dec. 31, 2015 USD ($)	Dec. 31, 2014
Concentrations of Credit Risk and Other Risks and Uncertainties
Number of major financial institutions with which the company's cash and cash equivalents are deposited \| Institution	1
Restricted Cash
Long term deposit consisting of letter of credit serving as security for lease	$ 603	$ 603
Headquarters and laboratory facilities, South San Francisco, Lease expiring 2016
Restricted Cash
Amount held as security for lease	120	118
Long term deposit consisting of letter of credit serving as security for lease		$ 603
Headquarters and laboratory facilities, South San Francisco, Lease signed in April 2015
Restricted Cash
Amount held as security for lease	603
Long term deposit consisting of letter of credit serving as security for lease	$ 603
Revenue \| Revenue concentration risk
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	47.00%	49.00%	55.00%
Revenue \| Revenue concentration risk \| Medicare
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	27.00%	26.00%	26.00%
Revenue \| Revenue concentration risk \| United Healthcare
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	12.00%	14.00%	18.00%
Revenue \| Revenue concentration risk \| Aetna
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	8.00%	9.00%	11.00%
Accounts receivable \| Gross receivables concentration risk \| Medicare
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	18.00%	31.00%
Accounts receivable \| Gross receivables concentration risk \| United Healthcare
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	8.00%	25.00%
Accounts receivable \| Gross receivables concentration risk \| Aetna
Concentrations of Credit Risk and Other Risks and Uncertainties
Concentrations of credit risk (as a percent)	4.00%	23.00%