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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	200,856	206,483	197,047	220,683
Granted (in shares)	1,789	0	93,853	97,873
Vested (in shares)	0	(687)	(75,683)	(89,681)
Forfeited (in shares)	(2,287)	0	(14,859)	(23,079)
Outstanding, end of period (in shares)	200,358	205,796	200,358	205,796
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 33.03	$ 31.50	$ 31.53	$ 29.57
Granted (in dollars per share)	35.61	0.00	34.12	33.47
Vested (in dollars per share)	0.00	24.48	30.56	28.84
Forfeited (in dollars per share)	32.83	0.00	32.35	31.50
Outstanding, end of period (in dollars per share)	$ 33.05	$ 31.53	$ 33.05	$ 31.53
Total weighted-average grant-date fair value	$ 0.1	$ 0.0	$ 3.2	$ 3.3
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	66,177	33,770	32,904	83,106
Granted (in shares)	878	0	37,819	37,204
Vested (in shares)	0	0	0	(83,106)
Forfeited (in shares)	(1,490)	0	(5,158)	(3,434)
Outstanding, end of period (in shares)	65,565	33,770	65,565	33,770
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.82	$ 39.51	$ 39.51	$ 22.95
Granted (in dollars per share)	38.20	0.00	38.21	39.51
Vested (in dollars per share)	0.00	0.00	0.00	22.95
Forfeited (in dollars per share)	38.85	0.00	38.84	39.51
Outstanding, end of period (in dollars per share)	$ 38.81	$ 39.51	$ 38.81	$ 39.51
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 1.4	$ 1.5
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	264,707	135,078	131,616	109,816
Granted (in shares)	3,511	0	151,277	148,818
Vested (in shares)	0	0	0	(109,816)
Forfeited (in shares)	(5,958)	0	(20,633)	(13,740)
Outstanding, end of period (in shares)	262,260	135,078	262,260	135,078
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 33.79	$ 33.47	$ 33.47	$ 25.18
Granted (in dollars per share)	35.58	0.00	34.12	33.47
Vested (in dollars per share)	0.00	0.00	0.00	25.18
Forfeited (in dollars per share)	33.80	0.00	33.80	33.48
Outstanding, end of period (in dollars per share)	$ 33.82	$ 33.47	$ 33.82	$ 33.47
Total weighted-average grant-date fair value	$ 0.1	$ 0.0	$ 5.2	$ 5.0