Collaboration and License Agreements - Baxalta Collaboration Agreement (Details) $ / shares in Units, $ in Thousands		1 Months Ended			3 Months Ended								12 Months Ended
	Jun. 03, 2016	Feb. 28, 2015 USD ($) deliverable	Jan. 31, 2015 USD ($) deliverable	Feb. 29, 2012 USD ($) collaboration_project	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) collaboration_project	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognition of upfront payments													$ 55,015	$ 28,351	$ 9,014
Research and development revenue					$ 51,239	$ 13,200	$ 4,430	$ 3,210	$ 18,378	$ 5,805	$ 5,738	$ 5,050
Baxalta Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Recognition of upfront payments														21,983	$ 9,014
Additional collaboration products | collaboration_project				4
Upfront payment agreed to be paid				$ 33,000
Option payment not to be received by entity				7,000
Option payments for additional product candidates				12,000
Royalty payment, percent of net sales	5.00%
One time cash payment									$ 51,200
Additional payment to be paid				28,000
Research and development revenue														22,000
Impact on performance period														$ 11,000
Change in performance period per share (in dollars per share) | $ / shares														$ 0.16
Accounting Standards Update 2009 to 2013 | Baxalta Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional number of follow-on biologic projects for research and development service | collaboration_project													4
Total arrangement consideration				61,000
Portion of revised arrangement consideration allocated to the first initial product license as a result of termination of product				10,300
Portion of revised arrangement consideration allocated to each of the three additional product licenses as a result of termination of product				10,300
Portion allocated to second initial product license				9,400
Portion of revised arrangement consideration allocated to the joint steering committee unit of accounting as a result of termination of product				$ 114,000
Significant deliverables, identified number | deliverable			7
Number of follow-on biologic projects for development license and product license		2											6
Consideration portion reallocated to remaining deliverables			$ 61,000
Decrease in consideration portion reallocated to remaining deliverables		$ 40,000