Stock compensation (Details 2) (Stock options, USD $)	6 Months Ended
	Jun. 30, 2013
Stock options
Shares Available For Grant
Outstanding at the beginning of the period (in shares)	5,406,277
Additional authorized (in shares)	5,200,000
Options granted (in shares)	(4,973,500)
Options cancelled (in shares)	257,756
Outstanding at the end of the period (in shares)	5,890,533
Number of Shares Outstanding
Outstanding at the beginning of the period (in shares)	21,816,611
Options granted (in shares)	4,973,500
Options exercised (in shares)	(2,833,583)
Options cancelled (in shares)	(258,040)
Outstanding at the end of the period (in shares)	23,698,488
Exercisable at the end of period (in shares)	14,837,911
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 12.05
Options granted (in dollars per share)	$ 18.65
Options exercised (in dollars per share)	$ 8.70
Options cancelled (in dollars per share)	$ 17.25
Outstanding at the end of the period (in dollars per share)	$ 13.78
Exercisable at the end of the period (in dollars per share)	$ 11.25