Stock compensation (Tables)

3 Months Ended

Mar. 31, 2013

Stock compensation

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

	Employee StockOptions For theThree MonthsEnded				Employee StockPurchase Plan For theThree MonthsEnded
	March 31,				March 31,
	2013		2012		2013		2012
Average risk-free interest rates	0.55	%	0.47	%	0.25	%	0.33	%
Average expected life (in years)	4.21		3.81		0.25		0.25
Volatility	47	%	58	%	44	%	44	%
Weighted-average fair value (in dollars)	6.95		7.71		3.26		3.21

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
Options granted	(4,577,500	)	4,577,500		18.31
Options exercised	—		(2,067,375	)	8.60
Options cancelled	117,789		(118,073	)	16.69
Balance at March 31, 2013	946,566		24,208,663		$	13.51
Exercisable, March 31, 2013	—		14,640,252		$	10.70