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Stockholders' equity - Restricted Stock Units Activity (Details) - RSUs [Member]
shares in Thousands
9 Months Ended
Sep. 30, 2015
$ / shares
shares
Shares  
Non-vested shares at beginning of period (shares) 4,307
Granted (shares) 1,525
Vested (shares) (1,372)
Forfeited (shares) (52)
Non-vested shares at end of period (shares) 4,408
Weighted- average grant date fair value  
Non-vested shares at beginning of period (in dollars per share) | $ / shares $ 21.98
Granted (in dollars per share) | $ / shares 52.93
Vested (in dollars per share) | $ / shares 16.53
Forfeited (in dollars per share) | $ / shares 69.51
Non-vested shares at end of period (in dollars per share) | $ / shares $ 33.82