Summary of Significant Accounting Policies - Segments (Details) - item	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Segment Information
Number of operating segments	1
CytomX \| Revenues percentage \| Customer concentration
Segment Information
Percentages of revenue recognized	4.00%	48.00%
Roche \| Revenues percentage \| Customer concentration
Segment Information
Percentages of revenue recognized	37.00%	27.00%
Sanofi \| Revenues percentage \| Customer concentration
Segment Information
Percentages of revenue recognized		21.00%
Takeda \| Revenues percentage \| Customer concentration
Segment Information
Percentages of revenue recognized	56.00%	3.00%

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=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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