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Share-based compensation - Summary of Restricted Share Awards and Performance Share Awards (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Restricted Share Awards      
Number of Shares      
Unvested balance at beginning of period (in shares) 2,157,336    
Granted (in shares) 1,076,619    
Vested (in shares) (924,366)    
Forfeited (in shares) (84,450)    
Unvested balance at end of period (in shares) 2,225,139 2,157,336  
Weighted-Average Grant Date Fair Value      
Unvested balance at beginning of period (in dollars per share) $ 9.03    
Granted (in dollars per share) 7.92 $ 7.25 $ 10.25
Vested (in dollars per share) 8.95    
Forfeited (in dollars per share) 8.89    
Unvested balance at end of period (in dollars per share) $ 8.53 $ 9.03  
Performance Share Awards      
Number of Shares      
Unvested balance at beginning of period (in shares) 913,407    
Granted (in shares) 552,395    
Forfeited (in shares) (265,088)    
Canceled (in shares) (1,200,714)    
Unvested balance at end of period (in shares) 0 913,407  
Weighted-Average Grant Date Fair Value      
Unvested balance at beginning of period (in dollars per share) $ 7.22    
Granted (in dollars per share) 6.38 $ 5.83 $ 8.53
Forfeited (in dollars per share) 7.99    
Canceled (in dollars per share) 6.66    
Unvested balance at end of period (in dollars per share) $ 0 $ 7.22