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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, 2017
Mar. 31, 2016
Restricted stock units    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 220,683 210,634
Granted (in shares) 96,977 94,282
Vested (in shares) (81,624) (78,379)
Forfeited (in shares) 0 0
Outstanding, end of period (in shares) 236,036 226,537
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 29.57 $ 28.82
Granted (in dollars per share) 33.48 29.90
Vested (in dollars per share) 28.85 27.92
Forfeited (in dollars per share) 0.00 0.00
Outstanding, end of period (in dollars per share) $ 31.42 $ 29.59
Total weighted-average grant-date fair value $ 3.2 $ 2.8
LTIP linked to TRS    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 83,106 162,500
Granted (in shares) 36,971 0
Vested (in shares) (83,106) (78,553)
Forfeited (in shares) 0 0
Outstanding, end of period (in shares) 36,971 83,947
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 22.95 $ 27.66
Granted (in dollars per share) 39.51 0.00
Vested (in dollars per share) 22.95 32.69
Forfeited (in dollars per share) 0.00 0.00
Outstanding, end of period (in dollars per share) $ 39.51 $ 22.95
Total weighted-average grant-date fair value $ 1.5 $ 0.0
LTIP awards linked to other performance conditions    
Restricted stock awards and restricted stock units    
Outstanding, beginning of period (in shares) 109,816 222,647
Granted (in shares) 147,888 0
Vested (in shares) (109,816) (109,097)
Forfeited (in shares) 0 0
Outstanding, end of period (in shares) 147,888 113,550
Weighted-average grant-date fair value per share    
Outstanding, beginning of period (in dollars per share) $ 25.18 $ 26.02
Granted (in dollars per share) 33.48 0.00
Vested (in dollars per share) 25.18 26.89
Forfeited (in dollars per share) 0.00 0.00
Outstanding, end of period (in dollars per share) $ 33.48 $ 25.18
Total weighted-average grant-date fair value $ 5.0 $ 0.0