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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 65,578 32,904 83,106
Granted (in shares) 35,215 37,832 37,204
Vested (issued or unissued and cancelled) (in shares) 0 0 (83,106)
Forfeited (in shares) (4,391) (5,158) (4,300)
Outstanding, end of period (in shares) 96,402 65,578 32,904
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) $ 38.81 $ 39.51 $ 22.95
Granted (in dollars per share) 41.07 38.21 39.51
Vested (issued or unissued and cancelled) (in dollars per share) 0 0 22.95
Forfeited (in dollars per share) 39.19 38.84 39.51
Outstanding, end of period (in dollars per share) $ 39.62 $ 38.81 $ 39.51
Total weighted-average grant-date fair value $ 1.4 $ 1.4 $ 1.5