Commitments and Contingencies (Details)		3 Months Ended		9 Months Ended
	Apr. 29, 2015 ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Operating Leases
Restricted cash, noncurrent		$ 603,000		$ 603,000		$ 118,000
Future minimum lease payments under non-cancelable operating leases
July through December 31, 2015		250,000		250,000
2016		1,822,000		1,822,000
2017		2,142,000		2,142,000
2018		2,102,000		2,102,000
2019		2,026,000		2,026,000
Thereafter		14,038,000		14,038,000
Total minimum lease payments		22,380,000		22,380,000
Facilities rent expense		687,000	$ 213,000	1,277,000	$ 639,000
Expected straight-line rent expense per quarter until new headquarters is utilized		500,000		500,000
Supplies Purchase Commitments
Non-cancelable purchase commitment		1,000,000		1,000,000
Headquarters and laboratory facilities, South San Francisco, Lease signed in April 2015
Operating Leases
Amount of space leased | ft²	59,000
Amount of funding provided by landlord for tenant improvements		3,300,000		3,300,000
Estimated amount of improvements expected in addition to landlord's tenant allowance		3,500,000		3,500,000
Tenant improvements receivable		1,700,000		1,700,000
Security deposit		603,000		603,000
Restricted cash, noncurrent		603,000		603,000
Laboratory facilities, Austin, Texas | Other assets
Operating Leases
Security deposit		$ 75,000		$ 75,000		$ 75,000