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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 83,106 162,500 257,956
Granted (in shares) 37,204 0 0
Vested (in shares) (83,106) (78,553) (75,915)
Forfeited (in shares) (4,300) (841) (19,541)
Outstanding, end of period (in shares) 32,904 83,106 162,500
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) $ 22.95 $ 27.66 $ 28.45
Granted (in dollars per share) 39.51 0.00 0.00
Vested (in dollars per share) 22.95 32.69 30.71
Forfeited (in dollars per share) 39.51 22.95 26.25
Outstanding, end of period (in dollars per share) $ 39.51 $ 22.95 $ 27.66
Total weighted-average grant-date fair value $ 1.5 $ 0.0 $ 0.0