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Share-based compensation - LTIP Awards Linked To Other Performance Conditions (Details) - LTIP awards linked to other performance conditions - 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) 109,816 222,647 364,731
Granted (in shares) 148,818 0 0
Vested (in shares) (109,816) (109,097) (121,249)
Increased above target (canceled) (in shares) 0 (1,989) 3,412
Forfeited (in shares) (17,202) (1,745) (24,247)
Outstanding, end of period (in shares) 131,616 109,816 222,647
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) $ 25.18 $ 26.02 $ 26.01
Granted (in dollars per share) 33.47 0.00 0.00
Vested (in dollars per share) 25.18 26.89 26.05
Increased above target (canceled) (in dollars per share) 0.00 25.26 26.89
Forfeited (in dollars per share) 33.48 25.19 25.82
Outstanding, end of period (in dollars per share) $ 33.47 $ 25.18 $ 26.02
Total weighted-average grant-date fair value $ 5.0 $ 0.0 $ 0.0