CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series G Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Series E Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series G Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series H Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series D Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series E Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital Series G Redeemable Convertible Preferred Stock	Additional Paid-In Capital Series D Redeemable Convertible Preferred Stock	Additional Paid-In Capital Series E Redeemable Convertible Preferred Stock	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Balance at beginning of period (in shares) at Dec. 31, 2016					448,686,791
Balance at beginning of period at Dec. 31, 2016					$ 1,176,661
Balance at end of period (in shares) at Dec. 31, 2017	448,686,791		63,291,156	81,428,340	448,686,791	0	0	63,291,156	81,428,340
Balance at end of period at Dec. 31, 2017	$ 1,176,661		$ 164,059	$ 501,880	$ 1,176,661	$ 0	$ 0	$ 164,059	$ 501,880
Balance at beginning of period (in shares) at Dec. 31, 2016										63,541,688
Balance at beginning of period at Dec. 31, 2016	(334,810)									$ 6	$ 31,305				$ (403)	$ (365,718)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										1,644,769
Exercise of options to purchase common stock (in shares)										20,542
Exercise of options to purchase common stock	212										212
Redeemable convertible preferred stock issuance costs	(152)										(152)
Stock-based compensation	40,055										40,314					(259)
Unrealized gain (loss) on marketable securities	(754)														(754)
Net loss	(255,916)															(255,916)
Balance at end of period (in shares) at Dec. 31, 2017										65,206,999
Balance at end of period at Dec. 31, 2017	(551,365)									$ 6	71,679				(1,157)	(621,893)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)						55,666,004	5,000,000
Issuance of redeemable convertible preferred stock, net of issuance costs		$ 51				$ 549,413	$ 111,546					$ 51
Repurchase of redeemable convertible preferred stock (in shares)								(269,180)	(544,100)
Repurchase of redeemable convertible preferred stock			$ (2,009)	$ (2,118)				$ (704)	$ (3,355)				$ (2,009)	$ (2,118)
Conversion of redeemable convertible preferred stock into common stock (in shares)					(508,539,515)	(55,666,004)	(5,000,000)	(63,021,976)	(80,884,240)	(236,012,913)
Conversion of redeemable convertible preferred stock into common stock	$ 1,833,561				$ (1,833,561)	$ (549,413)	$ (111,546)	$ (163,355)	$ (498,525)	$ 24	1,833,537
Balance at end of period (in shares) at Dec. 31, 2018	0				0	0	0	0	0
Balance at end of period at Dec. 31, 2018					$ 0	$ 0	$ 0	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										856,135
Exercise of options to purchase common stock (in shares)										446,864
Exercise of options to purchase common stock	$ 1,427										1,427
Conversion of redeemable convertible preferred stock into common stock (in shares)					508,539,515	55,666,004	5,000,000	63,021,976	80,884,240	236,012,913
Conversion of redeemable convertible preferred stock into common stock	1,833,561				$ (1,833,561)	$ (549,413)	$ (111,546)	$ (163,355)	$ (498,525)	$ 24	1,833,537
Proceeds of initial public offering, net of issuance costs of $41,322 (in shares)										26,275,993
Proceeds of initial public offering, net of issuance costs of $41,322	563,026									$ 3	563,023
Stock-based compensation	72,565										72,565
Unrealized gain (loss) on marketable securities	(163)														(163)
Net loss	(384,734)															(384,734)
Balance at end of period (in shares) at Dec. 31, 2018										328,798,904
Balance at end of period at Dec. 31, 2018	$ 1,530,241									$ 33	2,538,155				(1,320)	(1,006,627)
Balance at end of period (in shares) at Dec. 31, 2019	0				0
Balance at end of period at Dec. 31, 2019					$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Vesting of restricted common stock (in shares)										621,432
Exercise of options to purchase common stock (in shares)	6,945,306									6,945,306
Exercise of options to purchase common stock	$ 47,259									$ 1	47,258
Purchase of common stock under employee stock purchase plan (in shares)										171,343
Purchase of common stock under employee stock purchase plan	2,891										2,891
Stock-based compensation	81,122										81,122
Unrealized gain (loss) on marketable securities	3,124														3,124
Net loss	(514,021)															(514,021)
Balance at end of period (in shares) at Dec. 31, 2019										336,536,985
Balance at end of period at Dec. 31, 2019	$ 1,174,810									$ 34	$ 2,669,426				$ 1,804	$ (1,496,454)