Summary of Significant Accounting Policies - Effect of Adoption of ASC 606 on Condensed Consolidated Financial Statements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 60,209
Loss from operations	$ (130,556)	$ (129,815)	$ (143,709)	$ (141,640)	$ (153,031)	$ (85,818)	$ (97,015)	$ (77,402)	(545,720)	$ (413,266)	$ (269,356)
Loss before income taxes									(514,716)	(384,408)	(255,996)
Net loss									$ (514,021)	$ (384,734)	$ (255,916)
Net loss per share - basic and diluted (in dollars per share)	$ (0.37)	$ (0.37)	$ (0.41)	$ (0.40)	$ (1.14)	$ (1.32)	$ (1.43)	$ (1.16)	$ (1.55)	$ (4.95)	$ (4.18)
Accounts receivable	$ 5,032				$ 11,686				$ 5,032	$ 11,686		$ 8,948
Prepaid expenses and other current assets	19,403				28,399				19,403	28,399		28,099
Property and equipment, net					211,977					211,977		209,494
Right-of-use assets, operating leases	86,414								86,414			63,334
Assets	1,589,422				1,962,149				1,589,422	1,962,149		2,019,962
Deferred revenue, current	63,310				109,056				63,310	109,056		81,775
Other current liabilities	5,063				3,464				5,063	3,464		9,331
Deferred revenue, non-current	138,995				165,352				138,995	165,352		161,911
Operating lease liabilities, non-current	93,675								93,675			64,250
Financing lease liabilities, non-current	38,689								38,689			37,718
Deferred lease obligation, non-current					10,006					10,006
Lease financing obligation					33,489					33,489
Accumulated deficit	(1,496,454)				(1,006,627)				(1,496,454)	(1,006,627)		(982,432)
Total liabilities and stockholders’ equity	1,589,422				1,962,149				1,589,422	1,962,149		2,019,962
Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									94,978
Loss from operations									(510,951)
Loss before income taxes									(479,947)
Net loss									$ (479,252)
Net loss per share - basic and diluted (in dollars per share)									$ (1.45)
Deferred revenue, current	55,256								$ 55,256
Deferred revenue, non-current	143,007								143,007
Accumulated deficit	(1,489,669)								(1,489,669)
Accounting Standards Update 2016-02
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Right-of-use assets, operating leases												63,300
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable												$ (2,700)
Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									34,769
Loss from operations									34,769
Loss before income taxes									34,769
Net loss									$ 34,769
Net loss per share - basic and diluted (in dollars per share)									$ 0.10
Deferred revenue, current	(8,054)								$ (8,054)
Deferred revenue, non-current	4,012								4,012
Accumulated deficit	$ 6,785								6,785
Collaboration revenue
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									42,803
Collaboration revenue | Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									45,570
Collaboration revenue | Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									2,767
Collaboration revenue from related party
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									5,233
Collaboration revenue from related party | Balance without adoption of ASC 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									37,235
Collaboration revenue from related party | Accounting Standards Update 2014-09 | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total revenue									$ 32,002
Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable					11,686					11,686
Prepaid expenses and other current assets					28,399					28,399
Property and equipment, net					211,977					211,977
Assets					1,962,149					1,962,149
Deferred revenue, current					109,056					109,056
Other current liabilities					3,464					3,464
Deferred revenue, non-current					165,352					165,352
Deferred lease obligation, non-current					10,006					10,006
Lease financing obligation					33,489					33,489
Accumulated deficit					(1,006,627)					(1,006,627)
Total liabilities and stockholders’ equity					1,962,149					1,962,149
Adjustment | Accounting Standards Update 2016-02
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Prepaid expenses and other current assets					(300)					(300)
Property and equipment, net					(2,483)					(2,483)
Right-of-use assets, operating leases					63,334					63,334
Assets					60,551					60,551
Other current liabilities					5,867					5,867
Operating lease liabilities, non-current					64,250					64,250
Financing lease liabilities, non-current					37,718					37,718
Deferred lease obligation, non-current					(10,006)					(10,006)
Lease financing obligation					(33,489)					(33,489)
Accumulated deficit					(3,789)					(3,789)
Total liabilities and stockholders’ equity					60,551					60,551
Adjustment | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable					(2,738)					(2,738)
Assets					(2,738)					(2,738)
Deferred revenue, current					(27,281)					(27,281)
Deferred revenue, non-current					(3,441)					(3,441)
Accumulated deficit					27,984					27,984
Total liabilities and stockholders’ equity					$ (2,738)					$ (2,738)