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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, 2019
Mar. 31, 2018
Restricted stock units    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 200,358 197,047
Granted (in shares) 94,559 88,905
Vested (in shares) (76,712) (75,235)
Forfeited (in shares) (6,980) (2,629)
Outstanding, end of period (in shares) 211,225 208,088
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 33.05 $ 31.53
Granted (in dollars per share) 37.68 34.10
Vested (in dollars per share) 32.61 30.55
Forfeited (in dollars per share) 33.18 33.09
Outstanding, end of period (in dollars per share) $ 35.28 $ 32.97
Total weighted-average grant-date fair value $ 3.6 $ 3.0
LTIP linked to TRS    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 65,578 32,904
Granted (in shares) 34,647 35,626
Vested (in shares) 0 0
Forfeited (in shares) (1,914) (1,739)
Outstanding, end of period (in shares) 98,311 66,791
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 38.81 $ 39.51
Granted (in dollars per share) 41.07 38.21
Vested (in dollars per share) 0 0
Forfeited (in dollars per share) 38.62 38.83
Outstanding, end of period (in dollars per share) $ 39.61 $ 38.84
Total weighted-average grant-date fair value $ 1.4 $ 1.4
LTIP awards linked to other performance conditions    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 276,169 131,616
Granted (in shares) 138,580 142,509
Vested (in shares) 0 0
Forfeited (in shares) (7,659) (6,958)
Outstanding, end of period (in shares) 407,090 267,167
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 33.80 $ 33.47
Granted (in dollars per share) 37.68 34.10
Vested (in dollars per share) 0 0
Forfeited (in dollars per share) 33.91 33.81
Outstanding, end of period (in dollars per share) $ 35.12 $ 33.80
Total weighted-average grant-date fair value $ 5.2 $ 4.9