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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 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:
 
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
2,997

 
52.67

 
 
 
 
Exercised
(1,493
)
 
17.94

 
 
 
 
Forfeited or expired
(441
)
 
33.59

 
 
 
 
Outstanding as of December 31, 2018
13,789

 
$
31.73

 
7.36
 
$
146,270

Exercisable as of December 31, 2018
4,556

 
$
20.94

 
5.85
 
$
82,716

Options vested or expected to vest as of 
 December 31, 2018 (1)
12,499

 
$
30.60

 
7.23
 
$
141,643

(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:
 
Year Ended December 31,
 
2018
 
2017
 
2016
Risk-free rate
2.9
%
 
2.4
%
 
1.7
%
Expected dividend yield
0.35
%
 
0.40
%
 
0.83
%
Expected volatility
39
%
 
41
%
 
41
%
Expected term (in years)
5.4

 
5.4

 
5.6
Nonvested Restricted Stock Shares Activity [Table Text Block]
The following table summarizes the Company's restricted stock activity:
 
Shares (in thousands)
 
Weighted-Average Grant Fair Value
 
Aggregate Intrinsic Value (in thousands)
Nonvested as of December 31, 2017
20

 
$
17.03

 
 
Granted
—

 
—

 
 
Vested
(20
)
 
17.03

 
993

Forfeited or expired
—

 
—

 
 
Nonvested as of December 31, 2018
—

 
$
—

 
$
—
Stock-Based Compensation Expense
The following table presents the stock-based compensation expense by caption for each period presented on the Consolidated Statements of Operations (in thousands):
 
Year Ended December 31,
 
2018
 
2017
 
2016
Cost of revenue
$
2,447

 
$
1,881

 
$
1,052

Research, development, and engineering
14,578

 
11,022

 
6,271

Selling, general, and administrative
24,065

 
19,039

 
13,235

 
$
41,090

 
$
31,942

 
$
20,558