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Share-based compensation - Summary of changes in share based compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Restricted stock units    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 197,047 220,683
Granted (in shares) 88,905 96,977
Vested (in shares) (75,235) (81,624)
Forfeited (in shares) (2,629) 0
Outstanding, end of period (in shares) 208,088 236,036
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 31.53 $ 29.57
Granted (in dollars per share) 34.10 33.48
Vested (in dollars per share) 30.55 28.85
Forfeited (in dollars per share) 33.09 0.00
Outstanding, end of period (in dollars per share) $ 32.97 $ 31.42
Total weighted-average grant-date fair value $ 3.0 $ 3.2
LTIP linked to TRS    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 32,904 83,106
Granted (in shares) 35,626 36,971
Vested (in shares) 0 (83,106)
Forfeited (in shares) (1,739) 0
Outstanding, end of period (in shares) 66,791 36,971
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 39.51 $ 22.95
Granted (in dollars per share) 38.21 39.51
Vested (in dollars per share) 0.00 22.95
Forfeited (in dollars per share) 38.83 0.00
Outstanding, end of period (in dollars per share) $ 38.84 $ 39.51
Total weighted-average grant-date fair value $ 1.4 $ 1.5
LTIP awards linked to other performance conditions    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 131,616 109,816
Granted (in shares) 142,509 147,888
Vested (in shares) 0 (109,816)
Forfeited (in shares) (6,958) 0
Outstanding, end of period (in shares) 267,167 147,888
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 33.47 $ 25.18
Granted (in dollars per share) 34.10 33.48
Vested (in dollars per share) 0.00 25.18
Forfeited (in dollars per share) 33.81 0.00
Outstanding, end of period (in dollars per share) $ 33.80 $ 33.48
Total weighted-average grant-date fair value $ 4.9 $ 5.0