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Share-based compensation - LTIP Awards Linked To Other Performance Conditions (Details) - LTIP awards linked to other performance conditions - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 403,768 276,169 131,616
Granted (in shares) 98,522 140,855 151,328
Vested (in shares) (135,804) 0 0
Increased above target (cancelled) (in shares) (136,163) 4,314 13,858
Forfeited (in shares) (9,608) (17,570) (20,633)
Outstanding, end of period (in shares) 220,715 403,768 276,169
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Outstanding, beginning of period (in dollars per share) $ 35.15 $ 33.80 $ 33.47
Granted (in dollars per share) 48.10 37.78 34.12
Vested (in dollars per share) 33.48 0 0
Increased above target (cancelled) (in dollars per share) 36.44 33.53 33.49
Forfeited (in dollars per share) 38.36 34.66 33.80
Outstanding, end of period (in dollars per share) $ 41.03 $ 35.15 $ 33.80
Total weighted-average grant-date fair value $ 4.7 $ 5.3 $ 5.2