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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation, Restricted Shares [Roll Forward]      
Outstanding, beginning of period (in shares) 220,683 210,634 261,235
Granted (in shares) 97,873 114,431 85,772
Vested (in shares) (92,147) (85,003) (102,173)
Forfeited (in shares) (29,362) (19,379) (34,200)
Outstanding, end of period (in shares) 197,047 220,683 210,634
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) $ 29.57 $ 28.82 $ 25.77
Granted (in dollars per share) 33.47 29.70 33.69
Vested (in dollars per share) 28.88 27.84 25.67
Forfeited (in dollars per share) 31.57 29.82 27.09
Outstanding, end of period (in dollars per share) $ 31.53 $ 29.57 $ 28.82
Total weighted-average grant-date fair value $ 3.3 $ 3.4 $ 2.9