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Stockholders' equity and stock-based compensation - Restricted Stock Awards - Outstanding restricted stock awards (Details) - Restricted stock awards
$ / shares in Units, shares in Thousands, $ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Restricted stock awards  
Outstanding at the beginning of the period (in shares) | shares 3,169
Granted (in shares) | shares 3,193
Forfeited (in shares) | shares (126)
Vested (in shares) | shares (1,791)
Outstanding at the end of the period (in shares) | shares 4,445
Weighted-average grant-date fair value (per award)  
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 12.81
Granted (in dollars per share) | $ / shares 8.43
Forfeited (in dollars per share) | $ / shares 10.55
Vested (in dollars per share) | $ / shares 12.34
Outstanding at the end of the period (in dollars per share) | $ / shares $ 9.91
Equity instruments other than options, aggregate intrinsic value, vested | $ $ 15.6