Share-based compensation - Summary of changes in share based compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	203,441	211,225	207,641	200,358
Granted (in shares)	916	0	78,595	94,559
Vested (in shares)	0	0	(77,719)	(76,712)
Forfeited (in shares)	0	(2,600)	(4,160)	(9,580)
Outstanding, end of period (in shares)	204,357	208,625	204,357	208,625
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 40.67	$ 35.28	$ 35.36	$ 33.05
Granted (in dollars per share)	37.90	0	47.99	37.68
Vested (in dollars per share)	0	0	34.19	32.61
Forfeited (in dollars per share)	0	35.56	35.81	33.82
Outstanding, end of period (in dollars per share)	$ 40.65	$ 35.28	$ 40.65	$ 35.28
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 3.8	$ 3.6
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	90,616	98,311	96,402	65,578
Granted (in shares)	0	0	24,630	34,647
Vested (in shares)	0	0	(29,409)	0
Forfeited (in shares)	0	0	(1,007)	(1,914)
Outstanding, end of period (in shares)	90,616	98,311	90,616	98,311
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.08	$ 39.61	$ 39.62	$ 38.81
Granted (in dollars per share)	0	0	48.62	41.07
Vested (in dollars per share)	0	0	39.51	0
Forfeited (in dollars per share)	0	0	41.72	38.62
Outstanding, end of period (in dollars per share)	$ 42.08	$ 39.61	$ 42.08	$ 39.61
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 1.2	$ 1.4
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	336,344	407,090	403,768	276,169
Granted (in shares)	0	0	98,522	138,580
Vested (in shares)	0	0	(135,804)	0
Increase above target (in shares)	(38,821)	0	(64,932)	0
Forfeited (in shares)	0	0	(4,031)	(7,659)
Outstanding, end of period (in shares)	297,523	407,090	297,523	407,090
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.64	$ 35.12	$ 35.15	$ 33.80
Granted (in dollars per share)	0	0	48.10	37.68
Vested (in dollars per share)	0	0	33.48	0
Increase above target (in dollars per share)	34.12	0	34.12	0
Forfeited (in dollars per share)	0	0	39.67	33.91
Outstanding, end of period (in dollars per share)	$ 40.37	$ 35.12	$ 40.37	$ 35.12
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 4.7	$ 5.2