Stockholders' Equity (Details)	12 Months Ended
	Dec. 31, 2020 vote $ / shares shares	Dec. 31, 2019 $ / shares shares
Class of Stock [Line Items]
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Preferred stock, outstanding (in shares)	0	0
Total shares outstanding (in shares)	168,002,726	121,036,289
Percentage of class B shares representing number of class A shares	0.05
2002 Stock Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	1,265,656	1,856,154
2018 Equity Incentive Plan
Class of Stock [Line Items]
Shares reserved for future issuance (in shares)	20,233,754	17,233,144
2018 ESPP
Class of Stock [Line Items]
Shares reserved for future issuance (in shares)	2,587,874	3,030,407
Common Stock Warrant, Exercise Price of $27.78
Class of Stock [Line Items]
Warrant exercise price (in dollars per share) | $ / shares	$ 27.78
Warrants outstanding (in shares)	468,548	481,181
Common Stock Warrant, Exercise Price of $38.64
Class of Stock [Line Items]
Warrant exercise price (in dollars per share) | $ / shares	$ 38.64
Warrants outstanding (in shares)	12,940	12,940
Class A common stock
Class of Stock [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	600,000,000	600,000,000
Common stock, outstanding (in shares)	140,094,633	84,549,511
Voting rights per share | vote	1
Common stock, conversion ratio (in shares)	1
Class A common stock | 2018 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	5,210,823	5,998,406
Class A common stock | 2018 Equity Incentive Plan | Restricted Stock
Class of Stock [Line Items]
Number of outstanding options (in shares)	5,914,754	3,456,172
Class B common stock
Class of Stock [Line Items]
Common stock, par value (in dollars per share) | $ / shares	$ 0.0001	$ 0.0001
Common stock, authorized (in shares)	600,000,000	600,000,000
Common stock, outstanding (in shares)	27,908,093	36,486,778
Voting rights per share | vote	10
Class B common stock | 2002 Stock Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	1,265,656
Class B common stock | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	8,877,792	9,982,756
Class B common stock | 2012 Equity Incentive Plan | Restricted Stock
Class of Stock [Line Items]
Shares reserved for future issuance (in shares)	504,034	6,656,094