Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2019

Equity [Abstract]

Stock Option Activity

Stock option activity under the Plans is as follows:

	Outstanding options	Weighted- average exercise price		Weighted- average remaining contractual term (years)	Aggregate intrinsic value (thousands)
Balance as of December 31, 2017	18,701,267	$	5.73	7.3	$	29,728
Granted	6,824,057	12.68
Exercised	(1,727,899)	4.69			16,816
Cancelled	(1,784,828)	7.19
Balance as of December 31, 2018	22,012,597	7.85		7.1	439,382
Granted	1,790,074	17.71
Exercised	(6,465,360)	6.32			87,544
Cancelled	(1,653,290)	10.88
Balance as of December 31, 2019	15,684,021	9.28		6.3	170,847
Vested and exercisable as of December 31, 2018	12,462,693	5.75		5.6	274,883
Vested and expected to vest as of December 31, 2018	20,926,797	7.69		7.0	421,047
Vested and exercisable as of December 31, 2019	9,913,182	7.14		5.1	129,341
Vested and expected to vest as of December 31, 2019	15,197,994	9.16		6.3	167,439

Assumptions Used to Estimate the Fair Value of Stock Options

The following range of assumptions were used to estimate the fair value of stock options granted to employees:

	Year Ended December 31,
	2019	2018	2017
Expected dividend yield	—	—	—
Expected volatility	48.8 - 49.7%	43.5 - 48.2%	40.7 - 57.1%
Risk-free interest rate	1.32 - 2.58%	2.96 - 3.09%	1.92 - 2.1%
Expected term (years)	5.04 - 6.08	5.28 - 6.08	5.02 - 6.08

Assumptions Used to Estimate Purchase Rights under the ESPP

The following range of assumptions were used to estimate the purchase rights granted under the ESPP on the first day of the offering period:

	Year Ended
	December 31, 2019
Expected dividend yield	—
Expected volatility	43.26 - 58.89%
Risk-free interest rate	1.62 - 2.31%
Expected term (years)	0.5

Restricted Stock Unit Activity

Restricted stock activity for the year ended December 31, 2019 is presented as follows:

	Outstanding RSUs and RSAs	Weighted-average grant date fair value per share		Weighted- average remaining contractual term (years)	Aggregate intrinsic value (thousands)
Balance at December 31, 2017	802,900	$	8.65
Awarded	686,072	16.09
Released	(809,567)
Cancelled	(8,799)	31.79
Balance at December 31, 2018	670,606	24.71
Awarded	4,055,344	20.38
Released	(437,844)	21.16
Cancelled	(490,587)	25.21
Balance at December 31, 2019	3,797,519	20.44		1.8	$	76,596
Vested and expected to vest as of December 31, 2019	3,126,182	20.46		1.6	63,055
Vested and expected to vest as of December 31, 2018	532,623	24.80		1.7	14,812