Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]

Estimated Weighted-average Fair Value per Share of Options Granted

The following assumptions were used for each respective period to calculate our stock-based compensation for each stock option grant on the date of the grant:

	Year Ended December 31,
	2017		2016
Stock Options:
Expected volatility	39% - 42%		41% - 42%
Expected term (in years)	4.89		4.89 - 5.60
Risk-free interest rate	1.78% - 2.47%		1.18% - 1.87%
Dividend yield	—	%	—	%

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

The following assumptions were used to calculate our stock-based compensation for each stock purchase right granted under the 2012 ESPP:

	Year Ended December 31,
	2018		2017		2016
ESPP:
Expected volatility	26% - 31%		28% - 49%		31% - 49%
Expected term (in years)	0.50		0.50		0.50
Risk-free interest rate	1.15% - 2.22%		0.40% - 1.15%		0.17% - 0.47%
Dividend yield	—	%	—	%	—	%