Stock-Based Compensation Expense (Tables)

3 Months Ended

Mar. 31, 2019

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

The following table summarizes the Company’s stock option activity for the three-month period ended March 31, 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	2,761		51.49
Exercised	(717	)	20.03
Forfeited or expired	(139	)	41.73
Outstanding as of March 31, 2019	15,694		$	35.65	7.63	$	255,322
Exercisable as of March 31, 2019	6,289		$	24.86	6.17	$	166,524
Options vested or expected to vest as of March 31, 2019 (1)	13,860		$	34.21	7.46	$	244,429

(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
	March 31, 2019		April 1, 2018
Risk-free rate	2.7	%	2.9	%
Expected dividend yield	0.39	%	0.32	%
Expected volatility	37	%	39	%
Expected term (in years)	5.3		5.3

Nonvested Restricted Stock Shares Activity

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
	March 31, 2019		April 1, 2018
Cost of revenue	$	451	$	797
Research, development, and engineering	4,467		4,815
Selling, general, and administrative	7,363		7,582
	$	12,281	$	13,194