Summary of Significant Accounting Policies - Impact of Accounting Changes Upon Adoption of ASC 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Cumulative Effect of Accounting Changes
Deferred revenue – related party, current and net of current portions	$ 0				$ 61,513				$ 0	$ 61,513	$ 92,519	$ 124,184
Accumulated deficit	(1,843,475)				(1,516,105)				(1,843,475)	(1,516,105)	(1,104,633)	(758,605)
Consolidated Balance Sheets
Accounts receivable, net	21,328				8,952				21,328	8,952	5,076
Collaboration receivable – related party	2,123				1,539				2,123	1,539	2,462
Collaboration receivable – other	1,948				1,928				1,948	1,928	670
Total current assets	636,677				611,704				636,677	611,704	613,780
Total assets	852,952				890,741				852,952	890,741	858,457
Deferred revenue – related party	0				10,933				0	10,933	32,710
Total current liabilities	94,394				92,886				94,394	92,886	93,503
Deferred revenue, net of current portion – related party	0				50,580				0	50,580	59,809
Total liabilities	453,452				250,213				453,452	250,213	170,920
Total stockholders’ equity	399,500				640,528				399,500	640,528	687,537		$ 375,503
Total liabilities and stockholders’ equity	852,952				890,741				852,952	890,741	858,457
Consolidated Statements of Operations
Revenues	44,045	$ 34,706	$ 37,347	$ 87,098	35,440	$ 26,024	$ 26,221	$ 30,227	203,196	117,912	94,387
Research and development expense									367,470	410,894	341,324
Total cost and expenses											456,866
Loss from operations	$ (92,434)	$ (90,327)	$ (90,196)	$ (43,192)	$ (105,929)	$ (109,060)	$ (113,861)	$ (97,483)	(316,149)	(426,333)	(362,479)
Net loss									$ (327,370)	$ (411,472)	$ (346,028)
Net loss per share - basic and diluted (in usd per share)	$ (1.41)	$ (1.43)	$ (1.31)	$ (0.59)	$ (1.60)	$ (1.81)	$ (1.87)	$ (1.59)	$ (4.74)	$ (6.86)	$ (6.03)
Consolidated Statements of Comprehensive Loss
Comprehensive loss									$ (327,467)	$ (409,099)	$ (346,810)
Adjustments to reconcile net loss to net cash used in operating activities:
Accounts receivable, net									(12,376)	(3,876)	(5,076)
Collaboration receivable – related party									(584)	923	(14)
Collaboration receivable – other									(20)	(1,258)	(670)
Deferred revenue – related party									(61,513)	(31,006)	(31,665)
Product revenue, net
Consolidated Statements of Operations
Revenues									121,089	59,851	13,841
Collaboration revenue – related party
Consolidated Statements of Operations
Revenues									68,274	39,257	60,661
Collaboration revenue – other
Consolidated Statements of Operations
Revenues									$ 3,571	$ 8,262	12,670
Under Topic 605
Cumulative Effect of Accounting Changes
Deferred revenue – related party, current and net of current portions													163,640
Accumulated deficit											(1,142,867)		$ (798,061)
Consolidated Balance Sheets
Accounts receivable, net											5,076
Collaboration receivable – related party											2,462
Collaboration receivable – other											230
Total current assets											613,340
Total assets											858,017
Deferred revenue – related party											29,133
Total current liabilities											89,926
Deferred revenue, net of current portion – related party											101,180
Total liabilities											208,714
Total stockholders’ equity											649,303
Total liabilities and stockholders’ equity											858,017
Consolidated Statements of Operations
Revenues											92,280
Research and development expense											337,995
Total cost and expenses											453,537
Loss from operations											(361,257)
Net loss											$ (344,806)
Net loss per share - basic and diluted (in usd per share)											$ (6.01)
Consolidated Statements of Comprehensive Loss
Comprehensive loss											$ (345,588)
Adjustments to reconcile net loss to net cash used in operating activities:
Accounts receivable, net											(5,076)
Collaboration receivable – related party											(14)
Collaboration receivable – other											(230)
Deferred revenue – related party											(33,327)
Under Topic 605 | Product revenue, net
Consolidated Statements of Operations
Revenues											13,841
Under Topic 605 | Collaboration revenue – related party
Consolidated Statements of Operations
Revenues											58,994
Under Topic 605 | Collaboration revenue – other
Consolidated Statements of Operations
Revenues											12,230
ASU 2014-09 | Cumulative Effect and Effect of Change
Cumulative Effect of Accounting Changes
Deferred revenue – related party, current and net of current portions												(39,456)
Accumulated deficit											38,234	$ 39,456
Consolidated Balance Sheets
Accounts receivable, net											0
Collaboration receivable – related party											0
Collaboration receivable – other											440
Total current assets											440
Total assets											440
Deferred revenue – related party											3,577
Total current liabilities											3,577
Deferred revenue, net of current portion – related party											(41,371)
Total liabilities											(37,794)
Total stockholders’ equity											38,234
Total liabilities and stockholders’ equity											440
Consolidated Statements of Operations
Revenues											2,107
Research and development expense											3,329
Total cost and expenses											3,329
Loss from operations											(1,222)
Net loss											$ (1,222)
Net loss per share - basic and diluted (in usd per share)											$ (0.02)
Consolidated Statements of Comprehensive Loss
Comprehensive loss											$ (1,222)
Adjustments to reconcile net loss to net cash used in operating activities:
Accounts receivable, net											0
Collaboration receivable – related party											0
Collaboration receivable – other											(440)
Deferred revenue – related party											1,662
ASU 2014-09 | Cumulative Effect and Effect of Change | Product revenue, net
Consolidated Statements of Operations
Revenues											0
ASU 2014-09 | Cumulative Effect and Effect of Change | Collaboration revenue – related party
Consolidated Statements of Operations
Revenues											1,667
ASU 2014-09 | Cumulative Effect and Effect of Change | Collaboration revenue – other
Consolidated Statements of Operations
Revenues											$ 440