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Employee plans - Summary of Restricted Stock Unit Activity (Details) - Restricted Stock Units
shares in Thousands
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Restricted Stock Units  
Outstanding at beginning of period (in shares) | shares 1,454
Granted (in shares) | shares 1,042
Vested (in shares) | shares (555)
Forfeited and expired (in shares) | shares (253)
Outstanding at end of period (in shares) | shares 1,688
Weighted-Average Grant-Date Fair Value  
Outstanding at beginning of period (in dollars per share) | $ / shares $ 12.68
Granted (in dollars per share) | $ / shares 13.85
Vested (in dollars per share) | $ / shares 13.31
Forfeited and expired (in dollars per share) | $ / shares 11.88
Outstanding at end of period (in dollars per share) | $ / shares $ 13.39