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Stock-based Compensation (Tables)
 12 Months Ended
Dec. 29, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Allocated Employee Stock-Based Compensation Expense
The allocation of stock-based compensation for our equity incentive plans and our Employee Stock Purchase Plan (the “ESPP”) was as follows (in thousands):
 
Year Ended December 31,
 
2017
 
2016
 
2015
Research and development
$
7,569

 
$
9,366

 
$
11,691

Selling, general and administrative
16,369

 
13,546

 
10,286

Total stock-based compensation
$
23,938

 
$
22,912

 
$
21,977
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value
The weighted average grant-date fair value of our stock option grants and ESPP purchases were as follows:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Stock options
$
11.42

 
$
4.77

 
$
2.55

ESPP
$
6.00

 
$
2.17

 
$
1.20
Schedule of Fair Value of Employee Share-Based Payments Awards Stock Option Assumptions and Weighted Average Fair Values
The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Stock options:
 
 
 
 
 
Risk-free interest rate
1.98
%
 
1.15
%
 
1.22
%
Dividend yield
—
%
 
—
%
 
—
%
Volatility
59
%
 
76
%
 
93
%
Expected life
4.5 years

 
4.4 years

 
4.5 years

ESPP:
 
 
 
 
 
Risk-free interest rate
1.09
%
 
0.55
%
 
0.15
%
Dividend yield
—
%
 
—
%
 
—
%
Volatility
58
%
 
65
%
 
98
%
Expected life
6 months

 
6 months

 
6 months
Schedule of Fair Value of Employee Share-Based Payments Awards ESPP Assumptions and Weighted Average Fair Values
The grant-date fair value of employee stock option grants and ESPP purchases was estimated using the following assumptions:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Stock options:
 
 
 
 
 
Risk-free interest rate
1.98
%
 
1.15
%
 
1.22
%
Dividend yield
—
%
 
—
%
 
—
%
Volatility
59
%
 
76
%
 
93
%
Expected life
4.5 years

 
4.4 years

 
4.5 years

ESPP:
 
 
 
 
 
Risk-free interest rate
1.09
%
 
0.55
%
 
0.15
%
Dividend yield
—
%
 
—
%
 
—
%
Volatility
58
%
 
65
%
 
98
%
Expected life
6 months

 
6 months

 
6 months
Summary of All Stock Option Activity
Stock option activity for the year ended December 31, 2017 was as follows (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
2,166,110

 
$
23.43

 
 
 
 
Exercised
(4,469,203
)
 
$
3.91

 
 
 
 
Forfeited
(229,793
)
 
$
8.09

 
 
 
 
Expired
(258,333
)
 
$
9.91

 
 
 
 
Options outstanding at December 31, 2017
22,208,446

 
$
6.83

 
4.05 years
 
$
523,448

Exercisable at December 31, 2017
16,158,740

 
$
4.51

 
3.46 years
 
$
418,304
Summary of All RSU Activity
A summary of all RSU activity were as follows (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
2,137,817

 
$
24.60

 
 
 
 
Vested and released
(708,541
)
 
$
7.97

 
 
 
 
Forfeited
(136,077
)
 
$
11.48

 
 
 
 
Awards outstanding at December 31, 2017
3,762,990

 
$
17.76

 
1.95 years
 
$
114,395