Stock-Based Compensation Expense (Tables)
 3 Months Ended
Apr. 01, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
The following table summarizes the Company’s stock option activity for the three-month period ended April 1, 2018:
 
Shares
(in thousands)
 
Weighted-
Average
Exercise
Price
 
Weighted-
Average
Remaining
Contractual
Term (in years)
 
Aggregate
Intrinsic
Value
(in thousands)
Outstanding as of December 31, 2017
12,726

 
$
25.24

 
 
 
 
Granted
2,043

 
56.44

 
 
 
 
Exercised
(697
)
 
15.96

 
 
 
 
Forfeited or expired
(68
)
 
25.84

 
 
 
 
Outstanding as of April 1, 2018
14,004

 
$
30.25

 
7.9
 
$
317,169

Exercisable as of April 1, 2018
4,879

 
$
19.88

 
6.5
 
$
156,676

Options vested or expected to vest as of April 1, 2018 (1)
12,377

 
$
28.99

 
7.8
 
$
294,379

 

(1) In addition to the vested options, the Company expects a portion of the unvested options to vest at some point in the future. Options expected to vest are calculated by applying an estimated forfeiture rate to the unvested options.
Weighted-Average Assumptions Used in Estimating Fair Values of Stock Options Granted
The fair values of stock options granted in each period presented were estimated using the following weighted-average assumptions:
 
Three-months Ended
 
April 1, 2018
 
April 2, 2017
Risk-free rate
2.9
%
 
2.4
%
Expected dividend yield
0.32
%
 
0.39
%
Expected volatility
39
%
 
41
%
Expected term (in years)
5.3

 
5.3
Nonvested Restricted Stock Shares Activity
The following table summarizes the Company's restricted stock activity for the three-month period ended April 1, 2018:
 
Shares (in thousands)
 
Weighted-Average Grant Fair Value
 
Aggregate Intrinsic Value (in thousands)(1)
Nonvested as of December 31, 2017
20

 
$
17.03

 
 
Granted
—

 
—

 
 
Vested
—

 
—

 
 
Forfeited or expired
—

 
—

 
 
Nonvested as of April 1, 2018
20

 
$
17.03

 
$
1,040


(1) Fair market value as of April 1, 2018.
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs
The following table presents the stock-based compensation expense by caption for each period presented on the Consolidated Statements of Operations (in thousands):
 
Three-months Ended
 
April 1, 2018
 
April 2, 2017
Cost of revenue
$
797

 
$
430

Research, development, and engineering
4,815

 
2,610

Selling, general, and administrative
7,582

 
4,443

 
$
13,194

 
$
7,483