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Share-based compensation - Schedule of restricted shares with performance condition (Details) - Performance Shares - $ / shares
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Number of non-vested restricted shares (in shares)      
Outstanding, beginning of year (in shares) 1,890,529 2,001,048 1,855,379
Granted (in shares) 749,322 862,176 556,403
Forfeited (in shares) (372,476) (823,977) (294,977)
Vested (in shares) (504,440) (148,718) (115,757)
Outstanding, end of year (in shares) 1,762,935 1,890,529 2,001,048
Number of non-vested restricted shares probable of vesting      
Outstanding, beginning of year (in shares) 1,314,036 1,185,220 887,203
Granted (in shares) 499,542 574,784 370,931
Forfeited (in shares) 39,214 (290,552) (4,102)
Vested ( in shares) (504,440) (148,718) (115,757)
Change in estimated restricted shares considered probable of vesting (in shares) 132,656 (6,698) 46,945
Outstanding, end of year (in shares) 1,402,580 1,314,036 1,185,220
Weighted average grant data fair value (in dollars per share)      
Outstanding, beginning of year (in dollars per share) $ 12.43 $ 12.80 $ 12.60
Granted (in dollars per share) 9.46 10.95 14.01
Forfeited (in dollars per share) 12.12 11.70 13.98
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value 12.16 11.40 14.00
Change in estimated restricted shares considerable probable of vesting (in dollars per share) 11.28 12.92 13.35
Outstanding, end of year (in dollars per share) $ 10.98 $ 12.43 $ 12.80