Stock-Based Compensation Expense (Tables)

9 Months Ended

Sep. 30, 2018

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

The following table summarizes the Company’s stock option activity for the nine-month period ended September 30, 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	2,318		55.66
Exercised	(1,434	)	18.06
Forfeited or expired	(429	)	33.24
Outstanding as of September 30, 2018	13,181		$	31.11	7.48	$	329,301
Exercisable as of September 30, 2018	4,327		$	20.14	6.01	$	154,377
Options vested or expected to vest as of September 30, 2018 (1)	11,916		$	29.99	7.37	$	310,772

(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				Nine-months Ended
	September 30, 2018		October 1, 2017		September 30, 2018		October 1, 2017
Risk-free rate	2.9	%	2.4	%	2.9	%	2.4	%
Expected dividend yield	0.32	%	0.39	%	0.32	%	0.39	%
Expected volatility	39	%	41	%	39	%	41	%
Expected term (in years)	5.4		5.1		5.3		5.3

Nonvested Restricted Stock Shares Activity

The following table summarizes the Company's restricted stock activity for the nine-month period ended September 30, 2018:



	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 September 30, 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				Nine-months Ended
	September 30, 2018		October 1, 2017		September 30, 2018		October 1, 2017
Cost of revenue	$	544	$	520	$	1,898	$	1,404
Research, development, and engineering	3,197		2,765		11,166		8,090
Selling, general, and administrative	5,402		4,741		18,275		13,861
	$	9,143	$	8,026	$	31,339	$	23,355