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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	236,191	203,441	193,939	207,641
Granted (in shares)	4,894	916	132,492	78,595
Vested (in shares)	(292)	0	(79,280)	(77,719)
Forfeited (in shares)	(11,018)	0	(17,376)	(4,160)
Outstanding, end of period (in shares)	229,775	204,357	229,775	204,357
Total weighted-average grant-date fair value	$ 0.2	$ 0.0	$ 4.6	$ 3.8
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 37.91	$ 40.67	$ 40.89	$ 35.36
Granted (in dollars per share)	44.61	37.90	34.37	47.99
Vested (in dollars per share)	38.77	0	38.51	34.19
Forfeited (in dollars per share)	38.74	0	40.01	35.81
Outstanding, end of period (in dollars per share)	$ 38.02	$ 40.65	$ 38.02	$ 40.65
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	100,053	90,616	89,222	96,402
Granted (in shares)	1,533	0	45,743	24,630
Vested (in shares)	0	0	(32,355)	(29,409)
Forfeited (in shares)	(10,427)	0	(11,451)	(1,007)
Outstanding, end of period (in shares)	91,159	90,616	91,159	90,616
Total weighted-average grant-date fair value	$ 0.1	$ 0.0	$ 1.9	$ 1.2
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.89	$ 42.08	$ 42.10	$ 39.62
Granted (in dollars per share)	41.12	0	41.12	48.62
Vested (in dollars per share)	0	0	38.20	39.51
Forfeited (in dollars per share)	42.82	0	43.10	41.72
Outstanding, end of period (in dollars per share)	$ 42.87	$ 42.08	$ 42.87	$ 42.08
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	335,702	336,344	220,715	403,768
Granted (in shares)	6,133	0	182,977	98,522
Vested (in shares)	0	0	(43,155)	(135,804)
Increase above target (cancelled) (in shares)	15,881	(38,821)	1,277	(64,932)
Forfeited (in shares)	(41,711)	0	(45,809)	(4,031)
Outstanding, end of period (in shares)	316,005	297,523	316,005	297,523
Total weighted-average grant-date fair value	$ 0.3	$ 0.0	$ 6.3	$ 4.7
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.04	$ 39.64	$ 41.03	$ 35.15
Granted (in dollars per share)	44.49	0	34.33	48.10
Vested (in dollars per share)	0	0	34.12	33.48
Increase above target (in dollars per share)	42.92	34.12	31.71	34.12
Forfeited (in dollars per share)	38.27	0	38.82	39.67
Outstanding, end of period (in dollars per share)	$ 38.38	$ 40.37	$ 38.38	$ 40.37