Stock compensation (Tables)

12 Months Ended

Dec. 31, 2015

Stock compensation

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

							Employee Stock
	Employee Stock Options						Purchase Plan
	For the Year Ended						For the Year Ended
	December 31,						December 31,
	2015		2014		2013		2015		2014		2013
Average risk-free interest rates	1.35	%	1.21	%	0.62	%	0.81	%	0.55	%	0.27	%
Average expected life (in years)	5.03		4.54		4.27		0.50		0.50		0.50
Volatility	49	%	51	%	48	%	55	%	48	%	49	%
Weighted-average fair value (in dollars)	34.55		25.65		8.18		16.26		9.56		4.22