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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Allocated Employee Stock-Based Compensation Expenses
We recorded and allocated employee stock-based compensation expense for our equity incentive plans and our 2000 Employee Stock Purchase Plan (“ESPP”) as follows (in thousands):
 
Three Months Ended March 31,
 
2017
 
2016
Research and development expense
$
1,478

 
$
5,564

Selling, general and administrative expense
3,235

 
5,621

Total stock-based compensation expense
$
4,713

 
$
11,185
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value
We use the Black-Scholes Merton option pricing model to value our stock options and ESPP purchases. The weighted average grant-date fair value of our stock options and ESPP purchases was as follows:
 
Three Months Ended March 31,
 
2017
 
2016
Stock options
$
9.92

 
$
2.51

ESPP
$
3.71

 
$
2.31
Schedule of Fair Value of Employee Share-Based Payments Awards Estimated Using the Assumptions and Weighted Average Fair Values
The fair value of stock options and ESPP purchases was estimated using the following assumptions:
 
Stock Options
 
Three Months Ended March 31,
 
2017
 
2016
Risk-free interest rate
1.62
%
 
1.16
%
Dividend yield
—
%
 
—
%
Expected volatility
64
%
 
79
%
Expected life
4.0 years

 
4.3 years

 
Employee Stock Purchase Plan
 
Three Months Ended March 31,
 
2017
 
2016
Risk-free interest rate
0.62
%
 
0.51
%
Dividend yield
—
%
 
—
%
Expected volatility
68
%
 
81
%
Expected life
6 months

 
6 months
Summary of All Stock Option Activity
A summary of stock option activity for the three months ended March 31, 2017 is presented below (dollars in thousands, except per share amounts):
 
Shares
 
Weighted
Average
Exercise Price
 
Weighted
Average
Remaining Contractual
Term
 
Aggregate
Intrinsic
Value
Options outstanding at December 31, 2016
24,999,665

 
$
4.91

 
 
 
 
Granted
288,120

 
$
20.16

 
 
 
 
Exercised
(2,297,358
)
 
$
4.29

 
 
 
 
Forfeited
(141,021
)
 
$
7.23

 
 
 
 
Options outstanding at March 31, 2017
22,849,406

 
$
5.15

 
4.46 years
 
$
377,392

Exercisable at March 31, 2017
16,456,810

 
$
4.02

 
3.94 years
 
$
290,400
Summary of All RSU Activity
A summary of restricted stock unit (“RSU”) activity for the three months ended March 31, 2017 is presented below (dollars in thousands, except per share amounts):
 
Shares
 
Weighted
Average
Grant Date
Fair Value
 
Weighted
Average
Remaining
Contractual
Term
 
Aggregate
Intrinsic
Value
Awards outstanding at December 31, 2016
2,469,791

 
$
8.69

 
 
 
 
Awarded
135,750

 
$
20.06

 
 
 
 
Vested and released
(178,898
)
 
$
4.42

 
 
 
 
Forfeited
(73,495
)
 
$
10.36

 
 
 
 
Awards outstanding at March 31, 2017
2,353,148

 
$
9.62

 
1.87 years
 
$
50,993