Share-based compensation - Summary of changes in share based compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	206,483	225,752	220,683	210,634
Granted (in shares)	0	766	97,873	95,048
Vested (in shares)	(687)	(4,419)	(89,681)	(83,583)
Forfeited (in shares)	0	(2,352)	(23,079)	(2,352)
Outstanding, end of period (in shares)	205,796	219,747	205,796	219,747
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 31.50	$ 29.59	$ 29.57	$ 28.82
Granted (in dollars per share)	0.00	30.65	33.47	29.91
Vested (in dollars per share)	24.48	27.26	28.84	27.88
Forfeited (in dollars per share)	0.00	29.69	31.50	29.69
Outstanding, end of period (in dollars per share)	$ 31.53	$ 29.64	$ 31.53	$ 29.64
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 3.3	$ 2.8
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	33,770	83,947	83,106	162,500
Granted (in shares)	0	0	37,204	0
Vested (in shares)	0	0	(83,106)	(78,553)
Forfeited (in shares)	0	(175)	(3,434)	(175)
Outstanding, end of period (in shares)	33,770	83,772	33,770	83,772
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.51	$ 22.95	$ 22.95	$ 27.66
Granted (in dollars per share)	0.00	0.00	39.51	0.00
Vested (in dollars per share)	0.00	0.00	22.95	32.69
Forfeited (in dollars per share)	0.00	22.95	39.51	22.95
Outstanding, end of period (in dollars per share)	$ 39.51	$ 22.95	$ 39.51	$ 22.95
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 1.5	$ 0.0
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	135,078	113,550	109,816	222,647
Granted (in shares)	0	0	148,818	0
Vested (in shares)	0	0	(109,816)	(109,097)
Forfeited (in shares)	0	(699)	(13,740)	(699)
Outstanding, end of period (in shares)	135,078	112,851	135,078	112,851
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 33.47	$ 25.18	$ 25.18	$ 26.02
Granted (in dollars per share)	0.00	0.00	33.47	0.00
Vested (in dollars per share)	0.00	0.00	25.18	26.89
Forfeited (in dollars per share)	0.00	25.19	33.48	25.19
Outstanding, end of period (in dollars per share)	$ 33.47	$ 25.18	$ 33.47	$ 25.18
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 5.0	$ 0.0