Stockholders' Equity (Details)	9 Months Ended
	Sep. 30, 2020 shares $ / shares	Sep. 30, 2019 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)	153,796,572		121,036,289
Number of outstanding options (in shares)	24,198,742		27,949,582
Total number of outstanding options and warrants (in shares)	24,692,863		28,443,703
Total diluted shares (in shares)	178,489,435	149,479,992
Total plan shares, options and awards outstanding (in shares)	201,064,725		169,743,543
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)	1,595,345		1,856,154
2012 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	10,545,571		16,638,850
2018 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	12,057,826		9,454,578
Shares reserved for future issuance (in shares)	19,987,416		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
Class of Stock [Line Items]
Warrants outstanding (in shares)	494,121		494,121
Class B common stock
Class of Stock [Line Items]
Common stock, authorized (in shares)	600,000,000		600,000,000
Common stock, outstanding (in shares)	28,067,011		36,486,778
Voting rights per share	10
Class B common stock | 2002 Stock Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	1,595,345
Class B common stock | 2012 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	9,191,693
Class B common stock | Preferred stock warrants
Class of Stock [Line Items]
Warrant exercise price (in dollars per share) | $ / shares	$ 27.78		$ 38.64
Class B common stock | Common Stock Warrant
Class of Stock [Line Items]
Number of securities called by warrants (in shares)	481,181		12,940
Class A common stock
Class of Stock [Line Items]
Common stock, authorized (in shares)	600,000,000		600,000,000
Common stock, outstanding (in shares)	125,729,561		84,549,511
Voting rights per share	1
Common stock, conversion ratio	1
Class A common stock | 2018 Equity Incentive Plan
Class of Stock [Line Items]
Number of outstanding options (in shares)	5,817,989