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Stock-Based Compensation Expense (Tables)

6 Months Ended

Jun. 30, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

The following table summarizes the Company’s stock option activity for the six-month period ended June 30, 2019:



	Shares (in thousands)		Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding as of December 31, 2018	13,789		$	31.73
Granted	2,821		51.37
Exercised	(1,239	)	21.32
Forfeited or expired	(606	)	40.27
Outstanding as of June 30, 2019	14,765		$	36.00	7.42	$	207,599
Exercisable as of June 30, 2019	5,867		$	25.12	5.95	$	138,370
Options vested or expected to vest as of June 30, 2019 (1)	13,214		$	34.65	7.25	$	201,420

(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				Six-months Ended
	June 30, 2019		July 1, 2018		June 30, 2019		July 1, 2018
Risk-free rate	2.7	%	2.9	%	2.7	%	2.9	%
Expected dividend yield	0.39	%	0.32	%	0.39	%	0.32	%
Expected volatility	37	%	39	%	37	%	39	%
Expected term (in years)	5.2		5.4		5.3		5.3

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				Six-months Ended
	June 30, 2019		July 1, 2018		June 30, 2019		July 1, 2018
Cost of revenue	$	329	$	557	$	780	$	1,354
Research, development, and engineering	3,550		3,154		8,017		7,969
Selling, general, and administrative	7,088		5,291		14,451		12,873
	$	10,967	$	9,002	$	23,248	$	22,196