Summary of Significant Accounting Policies - Segments (Details) - item	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Information
Number of operating segments			1
Revenues percentage | Customer concentration
Segment Information
Percentages of revenue recognized	1.00%	2.00%	39.00%	2.00%
CytomX | Revenues percentage | Customer concentration
Segment Information
Percentages of revenue recognized	6.00%	3.00%	4.00%	22.00%
Novartis | Revenues percentage | Customer concentration
Segment Information
Percentages of revenue recognized	11.00%		4.00%
Roche | Revenues percentage | Customer concentration
Segment Information
Percentages of revenue recognized	78.00%	17.00%	50.00%	21.00%
Sanofi | Revenues percentage | Customer concentration
Segment Information
Percentages of revenue recognized		77.00%		53.00%