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Employee benefit plans - Restricted Stock Units Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Weighted-average grant date fair value      
Weighted average price of shares granted (usd per share) $ 4.59 $ 5.70 $ 5.83
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value $ 23,900,000 $ 34,900,000 $ 41,600,000
RSUs [Member]      
Shares (in thousands)      
Non-vested shares at beginning of period (shares) 8,225    
Granted (shares) 8,759    
Vested (shares) (3,962)    
Forfeited (shares) (2,383)    
Non-vested shares at end of period (shares) 10,639 8,225  
Weighted-average grant date fair value      
Non-vested shares at beginning of period (in dollars per share) $ 6.11    
Weighted average price of shares granted (usd per share) 4.59    
Weighted average price of shares vested (usd per share) 6.04    
Weighted average price of shares forfeited (usd per share) 5.40    
Non-vested shares at end of period (in dollars per share) $ 5.04 $ 6.11