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Share-based compensation - LTIP Linked to TRS (Details) - LTIP linked to TRS - 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) 96,402 65,578 32,904
Granted (in shares) 24,630 35,215 37,832
Vested (issued or unissued and cancelled) (in shares) (29,409) 0 0
Forfeited (in shares) (2,401) (4,391) (5,158)
Outstanding, end of period (in shares) 89,222 96,402 65,578
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) $ 39.62 $ 38.81 $ 39.51
Granted (in dollars per share) 48.62 41.07 38.21
Vested (issued or unissued and cancelled) (in dollars per share) 39.51 0 0
Forfeited (in dollars per share) 41.22 39.19 38.84
Outstanding, end of period (in dollars per share) $ 42.10 $ 39.62 $ 38.81
Total weighted-average grant-date fair value $ 1.2 $ 1.4 $ 1.4