CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ (DEFICIT) EQUITY - USD ($) $ in Thousands	Total	Revision of Prior Period, Adjustment	Previously Reported	Common Stock	Common Stock Revision of Prior Period, Adjustment	Common Stock Previously Reported	APIC	APIC Previously Reported	Note Receivable	Note Receivable Previously Reported	Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income Previously Reported	Accumulated Deficit	Accumulated Deficit Previously Reported
Beginning balance (in shares) at Dec. 31, 2019	68,776,614	1,048,328	67,728,286
Beginning balance at Dec. 31, 2019	$ 304,469		$ 304,469
Ending balance (in shares) at Jun. 30, 2020	68,776,614
Ending balance at Jun. 30, 2020	$ 304,469
Beginning balance (in shares) at Dec. 31, 2019				8,865,510	135,133	8,730,377
Beginning balance at Dec. 31, 2019	(243,009)		$ (243,009)	$ 1		$ 1	$ 3,816	$ 3,816	$ (408)	$ (408)	$ 60	$ 60	$ (246,478)	$ (246,478)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises and vested portion of common stock options (in shares)				20,452
Exercises and vested portion of stock options	50						50
Stock-based compensation	555						555
Interest on stock purchase promissory note	(6)								(6)
Unrealized net gain (loss) on marketable securities	31										31
Net loss	(25,787)												(25,787)
Ending balance (in shares) at Jun. 30, 2020				8,885,962
Ending balance at Jun. 30, 2020	$ (268,166)			$ 1			4,421		(414)		91		(272,265)
Ending balance (in shares) at Sep. 30, 2020	68,776,614
Ending balance at Sep. 30, 2020	$ 304,469
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises and vested portion of common stock options (in shares)				7,630
Exercises and vested portion of stock options	20						20
Stock-based compensation	426						426
Interest on stock purchase promissory note	(2)								(2)
Unrealized net gain (loss) on marketable securities	(64)										(64)
Net loss	(10,630)												(10,630)
Ending balance (in shares) at Sep. 30, 2020				8,893,592
Ending balance at Sep. 30, 2020	$ (278,416)			$ 1			4,867		(416)		27		(282,895)
Beginning balance (in shares) at Dec. 31, 2020	68,776,614	1,048,328	67,728,286
Beginning balance at Dec. 31, 2020	$ 304,469		$ 304,469
Convertible Preferred Stock
Exercise of convertible preferred stock warrants (in shares)	3,974,655
Exercise of convertible preferred stock warrants	$ 132,718
Conversion of preferred stock to common (in shares)	(72,751,269)
Conversion of preferred stock to common	$ (437,187)
Ending balance (in shares) at Jun. 30, 2021	0
Ending balance at Jun. 30, 2021	$ 0
Beginning balance (in shares) at Dec. 31, 2020				8,992,039	137,061	8,854,978
Beginning balance at Dec. 31, 2020	(361,496)		$ (361,496)	$ 1		$ 1	5,482	$ 5,482	(415)	$ (415)	11	$ 11	(366,575)	$ (366,575)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises and vested portion of common stock options (in shares)				1,089,670
Exercises and vested portion of stock options	2,175						2,175
Conversion of promissory note	0						(415)		415
RSUs withheld for tax purposes	(422)						(422)
Stock-based compensation	12,021						12,021
Unrealized net gain (loss) on marketable securities	(26)										(26)
Conversion of preferred stock to common (in shares)				72,751,269
Conversion of preferred stock to common	437,194			$ 7			437,187
Business Combination and PIPE financing (in shares)				43,894,156
Business Combination and PIPE financing	290,957			$ 4			290,953
Net loss	(144,887)												(144,887)
Ending balance (in shares) at Jun. 30, 2021				126,727,134
Ending balance at Jun. 30, 2021	$ 235,516			$ 12			746,981		0		(15)		(511,462)
Ending balance (in shares) at Sep. 30, 2021	0
Ending balance at Sep. 30, 2021	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercises and vested portion of common stock options (in shares)				5,968
Exercises and vested portion of stock options	46						46
401K match with MILE stock (in shares)				37,170
401K match with MILE stock	321						321
Stock-based compensation (in shares)				966,937
Stock-based compensation	7,929			$ 1			7,928
Net loss	(26,757)												(26,757)
Ending balance (in shares) at Sep. 30, 2021				127,737,209
Ending balance at Sep. 30, 2021	$ 217,055			$ 13			$ 755,276		$ 0		$ (15)		$ (538,219)