Stock-Based Compensation Expense (Tables)

12 Months Ended

Dec. 31, 2017

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

Effect of Recording Stock-Based Compensation Expense

The effect of recording stock-based compensation expense for the years ended December 31, 2017, 2016 and 2015 is as follows (in thousands): 

	Years Ended December 31,
	2017			2016			2015
Cost of revenue	$	—		$	—		$	—
Research, development and other related costs	13,277			7,104			4,005
Selling, general and administrative	20,185			13,997			7,512
Total stock-based compensation expense	33,462			21,101			11,517
Tax effect on stock-based compensation expense	(5,296		)	(6,314		)	(3,107		)
Net effect on net income	$	28,166		$	14,787		$	8,410

Stock-Based Compensation Expense Categorized by Equity Components

Stock-based compensation expense categorized by various equity components for the years ended December 31, 2017, 2016 and 2015 is summarized in the table below (in thousands):

	Years Ended December 31,
	2017		2016		2015
Employee stock options	$	1,980	$	3,249	$	2,676
Restricted stock awards and units	28,909		17,024		8,232
Employee stock purchase plan	2,573		828		609
Total stock-based compensation expense	$	33,462	$	21,101	$	11,517

Schedule of Assumptions Used To Value Options Granted

The following assumptions were used to value the awards granted: 

	Years Ended December 31,
	2017		2016		2015
Expected life (in years)	4.7		3.8		3.8
Risk-free interest rate	1.8	%	1.7	%	1.1 - 1.4%
Dividend yield	2.9	%	2.4	%	2.1 - 2.9%
Expected volatility	29.8	%	29.0	%	34.0 - 35.6%

The following assumptions were used to value the ESPP shares:

	Years Ended December 31,
	2017	2016		2015
Expected life (years)	2.0	2.0		2.0
Risk-free interest rate	1.2 - 1.3%	0.5 - 0.8%		0.4 - 0.7%
Dividend yield	2.0 - 2.5%	2.4 - 3.0%		2.1 - 3.4%
Expected volatility	28.3 - 30.8%	30.0	%	29.7 - 30.0%