Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adopting ASC 606					$ (5,511)					$ (5,511)
Income Statement [Abstract]
Research and development revenue	$ 42,822	$ 14,884	$ 13,031	$ 4,852	64,608	$ 24,090	$ 23,570	$ 26,614	$ 75,589	138,882	$ 109,619
Operating Income (Loss)	9,670	51,815	70,840	49,002	(12,633)	34,527	38,065	32,592	181,327	92,551	74,727
Net loss	8,245	50,300	69,885	47,631	(13,759)	33,188	36,908	31,759	176,061	88,096	21,003
Net loss	8,168	50,163	69,611	48,066	(13,572)	33,136	36,933	31,825	176,008	88,322	20,921
Statement of Financial Position [Abstract]
Deferred revenue, current	3,916				2,866				3,916	2,866
Deferred revenue, non-current	1,774				30,751				1,774	30,751
Accumulated deficit	743,826				562,254				743,826	562,254
Statement of Cash Flows [Abstract]
Operating Income (Loss)	9,670	51,815	70,840	49,002	(12,633)	34,527	38,065	32,592	181,327	92,551	74,727
Deferred revenue									33,438	5,015	(16,649)
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adopting ASC 606					(5,511)					(5,511)	(783)
Income Statement [Abstract]
Net loss									176,061	88,096	21,003
Calculated under Revenue Guidance in Effect before Topic 606
Income Statement [Abstract]
Operating Income (Loss)									189,775
Net loss									184,509
Net loss									184,456
Statement of Financial Position [Abstract]
Deferred revenue, current	6,186								6,186
Deferred revenue, non-current	2,442								2,442
Accumulated deficit	746,764								746,764
Statement of Cash Flows [Abstract]
Operating Income (Loss)									189,775
Deferred revenue									24,990
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Income Statement [Abstract]
Operating Income (Loss)									(8,448)
Net loss									(8,448)
Net loss									(8,448)
Statement of Financial Position [Abstract]
Deferred revenue, current	(2,270)								(2,270)
Deferred revenue, non-current	(668)								(668)
Accumulated deficit	(2,938)								(2,938)
Statement of Cash Flows [Abstract]
Operating Income (Loss)									(8,448)
Deferred revenue									8,448
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adopting ASC 606												$ 5,500
Research and development revenue
Income Statement [Abstract]
Research and development revenue	$ 32,059	$ 1,263	$ 1,252	$ 1,331	$ 51,239	$ 13,200	$ 4,430	$ 3,210	35,905	$ 72,079	$ 34,971
Research and development revenue | Calculated under Revenue Guidance in Effect before Topic 606
Income Statement [Abstract]
Research and development revenue									27,457
Research and development revenue | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Income Statement [Abstract]
Research and development revenue									$ 8,448