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Common Stock and Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2017
Equity [Abstract]  
Summary of Stock Option and RSU Activity
A summary of our stock option and RSU activity for the year ended December 31, 2017 is as follows (in thousands, except per share information):
 
 
Options Outstanding
 
RSUs Outstanding
Shares
Available
for Grant
 
Number of
Shares
 
Weighted
Average
Exercise Price
 
Weighted
Average
Remaining
Contractual
Term
 
Aggregate
Intrinsic
Value
 
Outstanding
RSUs
 
Weighted
Average
Grant Date
Fair Value
 
 
 
 
 
 
(In years)
 
 
 
 
 
 
Outstanding — January 1, 2017
6,039

 
8,479

 
$
14.52

 
7.49
 
$
66,449

 
6,936

 
$
20.81

Increase in authorized shares
4,833

 
 

 
 

 
 
 
 

 
 

 
 

Stock options granted
(1,203
)
 
1,203

 
28.14

 
 
 
 

 
 

 
 

RSUs granted
(3,647
)
 
 

 
 

 
 
 
 

 
3,647

 
28.09

Stock options exercised
 

 
(2,664
)
 
11.97

 
 
 
 

 
 

 
 

RSUs vested
 

 
 

 
 

 
 
 
 

 
(3,145
)
 
20.62

Stock options forfeited or canceled
579

 
(579
)
 
21.49

 
 
 
 

 
 

 
 

Stock options forfeited or canceled and unavailable for grant
—

 
(200
)
 
23.44

 
 
 
 
 
 
 
 
RSUs forfeited or canceled
1,400

 
 

 
 

 
 
 
 

 
(1,400
)
 
22.39

RSUs forfeited or canceled and unavailable for grant
—

 
 
 
 
 
 
 
 
 
(211
)
 
23.44

Outstanding — December 31, 2017
8,001

 
6,239

 
$
17.31

 
7.11
 
$
103,380

 
5,827

 
$
25.00

Options vested and expected to vest as of December 31, 2017
 
 
5,910

 
$
16.95

 
7.04
 
$
99,958

 
 
 
 
Options vested and exercisable as of December 31, 2017
 
 
3,243

 
$
14.01

 
6.40
 
$
64,311

 
 
 
 
Assumptions Used to Estimate Fair Value of Stock Options Granted to Employees
The assumptions used to estimate the fair value of stock options granted to employees are as follows:
 
Year Ended December 31,
2017
 
2016
 
2015
Expected volatility
44% - 48%
 
47% - 49%
 
49% - 54%
Dividend rate
0%
 
0%
 
0%
Risk-free interest rate
1.9% - 2.2%
 
1.1% - 2.0%
 
1.4% - 2.0%
Expected term (in years)
6.02 - 6.08
 
6.02 - 6.08
 
6.02 - 6.08
Assumptions Used to Estimate Fair Value of ESPP Awards
The assumptions used to estimate the fair value of ESPP awards are as follows:
 
Year Ended December 31,
 
2017
 
2016
Expected volatility
37% - 44%
 
42% - 48%
Dividend rate
0%
 
0%
Risk-free interest rate
1.02% - 1.62%
 
0.38% - 0.90%
Expected term (in years)
0.50 -1.50
 
0.50 -1.50