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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 276,169 131,616 109,816
Granted (in shares) 140,855 151,328 148,818
Vested (in shares) 0 0 (109,816)
Increased above target (cancelled) (in shares) 4,314 13,858 0
Forfeited (in shares) (17,570) (20,633) (17,202)
Outstanding, end of period (in shares) 403,768 276,169 131,616
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) $ 33.80 $ 33.47 $ 25.18
Granted (in dollars per share) 37.78 34.12 33.47
Vested (in dollars per share) 0 0 25.18
Increased above target (cancelled) (in dollars per share) 33.53 33.49 0
Forfeited (in dollars per share) 34.66 33.80 33.48
Outstanding, end of period (in dollars per share) $ 35.15 $ 33.80 $ 33.47
Total weighted-average grant-date fair value $ 5.3 $ 5.2 $ 5.0