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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	229,775	204,357	193,939	207,641
Granted (in shares)	1,432	0	133,924	78,595
Vested (in shares)	(343)	0	(79,623)	(77,719)
Forfeited (in shares)	(1,074)	0	(18,450)	(4,160)
Outstanding, end of period (in shares)	229,790	204,357	229,790	204,357
Total weighted-average grant-date fair value	$ 0.1	$ 0.0	$ 4.6	$ 3.8
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.02	$ 40.65	$ 40.89	$ 35.36
Granted (in dollars per share)	45.09	0	34.49	47.99
Vested (in dollars per share)	37.83	0	38.51	34.19
Forfeited (in dollars per share)	38.52	0	39.92	35.81
Outstanding, end of period (in dollars per share)	$ 38.06	$ 40.65	$ 38.06	$ 40.65
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	91,159	90,616	89,222	96,402
Granted (in shares)	281	0	46,024	24,630
Vested (in shares)	0	0	(32,355)	(29,409)
Forfeited (in shares)	(466)	0	(11,917)	(1,007)
Outstanding, end of period (in shares)	90,974	90,616	90,974	90,616
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 1.9	$ 1.2
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.87	$ 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)	42.49	0	43.07	41.72
Outstanding, end of period (in dollars per share)	$ 42.86	$ 42.08	$ 42.86	$ 42.08
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	316,005	297,523	220,715	403,768
Granted (in shares)	1,125	0	184,102	98,522
Vested (in shares)	0	0	(43,155)	(135,804)
Increase above target (cancelled) (in shares)	(22,354)	(21,807)	(21,077)	(86,739)
Forfeited (in shares)	(1,865)	0	(47,674)	(4,031)
Outstanding, end of period (in shares)	292,911	275,716	292,911	275,716
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 6.3	$ 4.7
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.38	$ 40.37	$ 41.03	$ 35.15
Granted (in dollars per share)	41.12	0	34.37	48.10
Vested (in dollars per share)	0	0	34.12	33.48
Increase above target (in dollars per share)	37.81	34.11	38.18	34.12
Forfeited (in dollars per share)	36.77	0	38.74	39.67
Outstanding, end of period (in dollars per share)	$ 38.45	$ 40.86	$ 38.45	$ 40.86