Summary of Significant Accounting Policies - Accounting Policies Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) segment shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Jan. 01, 2019 USD ($)	Dec. 11, 2018 shares
Accounting Policies [Abstract]
Number of operating segments | segment	1
Impairment charges related to marketable securities	$ 0	$ 0	$ 0
Allowance for doubtful accounts	0	0
Bad debt expense	0	0	0
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of long-lived assets	0	0	$ 0
Decrease in deferred tax asset	(507,268,000)	(323,501,000)
Valuation allowance	470,753,000	308,273,000
Remaining deferred revenues	199,528,000	$ 240,924,000
Right-of-use assets, operating leases	86,414,000			$ 63,334,000
Operating lease, liability	$ 97,259,000
Redeemable convertible preferred stock, shares issued | shares	0	0	448,686,791		508,539,515
Redeemable convertible preferred stock, shares outstanding | shares	0	0	448,686,791		508,539,515
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt instrument term	12 months
Marketable securities maturity term	12 months
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities maturity term	12 months
Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				27,984,000
Decrease in deferred tax asset				8,400,000
Valuation allowance				(8,400,000)
Increase (decrease) in deferred revenue				(30,700,000)
Accounting Standards Update 2016-02
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				(3,790,000)
Right-of-use assets, operating leases				63,300,000
Operating lease, liability				70,700,000
Accumulated Deficit | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				27,984,000
Accumulated Deficit | Accounting Standards Update 2016-02
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in cumulative effect of applying new standard				(3,790,000)
Combined 2018 AZ Agreements
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	$ 73,700,000	$ 115,600,000		75,700,000
Combined 2018 AZ Agreements | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				39,900,000
PCVSAV Agreement
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	83,800,000	111,300,000		125,200,000
PCVSAV Agreement | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				13,900,000
VEGF Exercise 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining deferred revenues	41,200,000	41,200,000		37,100,000
VEGF Exercise 2016 | Accounting Standards Update 2014-09
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in deferred revenue				$ 4,300,000
IPO | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock converted into common stock (in shares) | shares					236,012,913
Fair Value, Inputs, Level 1 | Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Letters of credit	$ 11,800,000	$ 12,100,000