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Share-based compensation - Restricted Stock Units (Details) - Restricted stock units - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 197,047 220,683 210,634
Granted (in shares) 93,853 97,873 114,431
Vested (in shares) (75,683) (92,147) (85,003)
Forfeited (in shares) (14,859) (29,362) (19,379)
Outstanding, end of period (in shares) 200,358 197,047 220,683
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) $ 31.53 $ 29.57 $ 28.82
Granted (in dollars per share) 34.12 33.47 29.70
Vested (in dollars per share) 30.56 28.88 27.84
Forfeited (in dollars per share) 32.35 31.57 29.82
Outstanding, end of period (in dollars per share) $ 33.05 $ 31.53 $ 29.57
Total weighted-average grant-date fair value $ 3.2 $ 3.3 $ 3.4