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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	208,625	200,856	200,358	197,047
Granted (in shares)	1,006	1,789	95,565	93,853
Vested (in shares)	(101)	0	(76,813)	(75,683)
Forfeited (in shares)	(2,889)	(2,287)	(12,469)	(14,859)
Outstanding, end of period (in shares)	206,641	200,358	206,641	200,358
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 35.28	$ 33.03	$ 33.05	$ 31.53
Granted (in dollars per share)	44.16	35.61	37.75	34.12
Vested (in dollars per share)	36.27	0	32.61	30.56
Forfeited (in dollars per share)	35.44	32.83	34.20	32.35
Outstanding, end of period (in dollars per share)	$ 35.32	$ 33.05	$ 35.32	$ 33.05
Total weighted-average grant-date fair value	$ 0.0	$ 0.1	$ 3.6	$ 3.2
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	98,311	66,177	65,578	32,904
Granted (in shares)	568	878	35,215	37,819
Vested (in shares)	0	0	0	0
Forfeited (in shares)	(2,477)	(1,490)	(4,391)	(5,158)
Outstanding, end of period (in shares)	96,402	65,565	96,402	65,565
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.61	$ 38.82	$ 38.81	$ 39.51
Granted (in dollars per share)	41.07	38.20	41.07	38.21
Vested (in dollars per share)	0	0	0	0
Forfeited (in dollars per share)	39.64	38.85	39.19	38.84
Outstanding, end of period (in dollars per share)	$ 39.62	$ 38.81	$ 39.62	$ 38.81
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 1.4	$ 1.4
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	407,090	264,707	276,169	131,616
Granted (in shares)	2,275	3,511	140,855	151,277
Vested (in shares)	0	0	0	0
Increase above target (in shares)	11,131	0	11,131	0
Forfeited (in shares)	(9,911)	(5,958)	(17,570)	(20,633)
Outstanding, end of period (in shares)	410,585	262,260	410,585	262,260
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 35.12	$ 33.79	$ 33.80	$ 33.47
Granted (in dollars per share)	44.05	35.58	37.78	34.12
Vested (in dollars per share)	0	0	0	0
Increase above target (in dollars per share)	33.49	0	33.49	0
Forfeited (in dollars per share)	35.24	33.80	34.66	33.80
Outstanding, end of period (in dollars per share)	$ 35.12	$ 33.82	$ 35.12	$ 33.82
Total weighted-average grant-date fair value	$ 0.1	$ 0.1	$ 5.3	$ 5.2