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Employee plans - Summary of Restricted Stock Unit Activity (Details) - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Weighted-Average Grant-Date Fair Value    
Granted (in dollars per share) $ 16.58 $ 15.20
Restricted Stock Units    
Restricted Stock Units    
Outstanding at beginning of period (in shares) 2,151  
Granted (in shares) 1,321  
Vested (in shares) (1,129)  
Forfeited and expired (in shares) (303)  
Outstanding at end of period (in shares) 2,040 2,151
Weighted-Average Grant-Date Fair Value    
Outstanding at beginning of period (in dollars per share) $ 12.90  
Granted (in dollars per share) 15.03  
Vested (in dollars per share) 13.23  
Forfeited and expired (in dollars per share) 13.81  
Outstanding at end of period (in dollars per share) $ 14.17 $ 12.90