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Share-based compensation - Schedule of restricted shares with performance condition (Details) - Performance Shares - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Number of non-vested restricted shares (in shares)      
Outstanding, beginning of year (in shares) 2,001,048 1,855,379 1,381,740
Granted (in shares) 862,176 556,403 935,825
Forfeited (in shares) (823,977) (294,977) (325,568)
Vested (in shares) (148,718) (115,757) (136,618)
Outstanding, end of year (in shares) 1,890,529 2,001,048 1,855,379
Number of non-vested restricted shares probable of vesting      
Outstanding, beginning of year (in shares) 1,185,220 887,203 577,486
Granted (in shares) 574,784 370,931 623,882
Forfeited (in shares) 290,552 (4,102) (45,617)
Vested ( in shares) $ (148,718) $ (115,757) $ (136,618)
Change in estimated restricted shares considered probable of vesting (in shares) (6,698) 46,945 (131,930)
Outstanding, end of year (in shares) 1,314,036 1,185,220 887,203
Weighted average grant data fair value (in dollars per share)      
Outstanding, beginning of year (in dollars per share) $ 12.80 $ 12.60 $ 12.91
Granted (in dollars per share) 10.95 14.01 12.66
Forfeited (in dollars per share) 11.70 13.98 12.57
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value 11.40 14.00 14.60
Change in estimated restricted shares considerable probable of vesting (in dollars per share) 12.92 13.35 12.17
Outstanding, end of year (in dollars per share) $ 12.43 $ 12.80 $ 12.60