Stock compensation (Tables)

6 Months Ended

Jun. 30, 2013

Stock compensation

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

	Employee Stock Options								Employee Stock Purchase Plan
	For theThreeMonthsEnded				For theSixMonthsEnded				For theThreeMonthsEnded				For theSixMonthsEnded
	June 30,								June 30,
	2013		2012		2013		2012		2013		2012		2013		2012
Average risk-free interest rates	0.72	%	0.60	%	0.57	%	0.49	%	0.36	%	0.33	%	0.26	%	0.29	%
Average expected life (in years)	4.71		4.36		4.25		3.86		0.50		0.50		0.50		0.50
Volatility	47	%	52	%	47	%	58	%	45	%	45	%	43	%	49	%
Weighted-average fair value (in dollars)	8.91		8.82		7.10		7.82		3.11		3.11		3.33		3.47

Schedule of activity summarized under all stock option plans

	SharesAvailable forGrant		NumberOutstanding		WeightedAverageExercisePrice perShare
Balance at December 31, 2012	5,406,277		21,816,611		$	12.05
Additional authorized	5,200,000		—		—
Options granted	(4,973,500	)	4,973,500		18.65
Options exercised	—		(2,833,583	)	8.70
Options cancelled	257,756		(258,040	)	17.25
Balance at June 30, 2013	5,890,533		23,698,488		$	13.78
Exercisable, June 30, 2013	—		14,837,911		$	11.25