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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	208,953	229,775	233,448	193,939
Granted (in shares)	0	1,432	98,463	133,924
Vested (in shares)	(4,244)	(343)	(95,658)	(79,623)
Forfeited (in shares)	(16,540)	(1,074)	(48,084)	(18,450)
Outstanding, end of period (in shares)	188,169	229,790	188,169	229,790
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.1	$ 4.1	$ 4.6
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.71	$ 38.02	$ 38.10	$ 40.89
Granted (in dollars per share)	0	45.09	41.31	34.49
Vested (in dollars per share)	39.49	37.83	37.73	38.51
Forfeited (in dollars per share)	39.99	38.52	39.19	39.92
Outstanding, end of period (in dollars per share)	$ 39.69	$ 38.06	$ 39.69	$ 38.06
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	76,730	91,159	90,974	89,222
Granted (in shares)	0	281	26,469	46,024
Vested (in shares)	0	0	(29,042)	(32,355)
Forfeited (in shares)	(4,829)	(466)	(16,500)	(11,917)
Outstanding, end of period (in shares)	71,901	90,974	71,901	90,974
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 1.5	$ 1.9
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 47.74	$ 42.87	$ 42.86	$ 42.10
Granted (in dollars per share)	0	41.12	54.92	41.12
Vested (in dollars per share)	0	0	41.07	38.20
Forfeited (in dollars per share)	48.52	42.49	44.33	43.07
Outstanding, end of period (in dollars per share)	$ 47.69	$ 42.86	$ 47.69	$ 42.86
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	293,711	316,005	306,342	220,715
Granted (in shares)	0	1,125	105,860	184,102
Vested (in shares)	0	0	(71,807)	(43,155)
Increase above target (cancelled) (in shares)	0	(22,354)	0	(21,077)
Forfeited (in shares)	(19,316)	(1,865)	(66,000)	(47,674)
Outstanding, end of period (in shares)	274,395	292,911	274,395	292,911
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 4.4	$ 6.3
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.91	$ 38.38	$ 38.42	$ 41.03
Granted (in dollars per share)	0	41.12	41.31	34.37
Vested (in dollars per share)	0	0	37.68	34.12
Increase above target (in dollars per share)	0	37.81	0	38.18
Forfeited (in dollars per share)	38.60	36.77	37.31	38.74
Outstanding, end of period (in dollars per share)	$ 40.00	$ 38.45	$ 40.00	$ 38.45