Stock Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock Compensation

Schedule of valuation assumptions used for valuation of fair value of stock compensation granted

	Employee Stock Options						Employee Stock Purchase Plan
	For the Year Ended						For the Year Ended
	December 31,						December 31,
	2017		2016		2015		2017		2016		2015
Average risk-free interest rates	1.80	%	1.30	%	1.35	%	1.53	%	0.90	%	0.81	%
Average expected life (in years)	5.25		4.99		5.03		0.50		0.50		0.50
Volatility	49	%	50	%	49	%	38	%	48	%	55	%
Weighted-average fair value (in dollars)	53.41		39.35		34.55		19.42		18.82		16.26