Stock compensation (Details 2) (Stock options, USD $)	3 Months Ended
	Mar. 31, 2013
Stock options
Shares Available For Grant
Outstanding at the beginning of the period (in shares)	5,406,277
Options granted (in shares)	(4,577,500)
Options cancelled (in shares)	117,789
Outstanding at the end of the period (in shares)	946,566
Number of Shares Outstanding
Outstanding at the beginning of the period (in shares)	21,816,611
Options granted (in shares)	4,577,500
Options exercised (in shares)	(2,067,375)
Options cancelled (in shares)	(118,073)
Outstanding at the end of the period (in shares)	24,208,663
Exercisable at the end of period (in shares)	14,640,252
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)	$ 18.31
Options exercised (in dollars per share)	$ 8.60
Options cancelled (in dollars per share)	$ 16.69
Outstanding at the end of the period (in dollars per share)	$ 13.51
Exercisable at the end of the period (in dollars per share)	$ 10.70