Net Loss Per Share Attributable to Common Shareholders - Weighted-Average Common Stock Equivalents were Excluded From Calculation of Diluted Net Loss Per Share Attributable to Common Shareholders (Details) - shares	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	110,758,398	107,842,523	109,993,511	107,194,266
Convertible Preferred Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	93,028,311	92,768,158	92,973,101	92,751,261
Options | 2009 Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	17,591,720	14,597,833	16,826,833	13,927,507
Options | 2008 Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	26,460	364,625	81,670	403,591
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	55,032	55,032	55,032	55,032
Convertible Preferred Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Convertible preferred stock (on as if converted basis)	56,875	56,875	56,875	56,875