Share-based compensation - Changes in Share Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	199,130	208,953	182,528	233,448
Granted (in shares)	0	0	100,088	98,463
Vested (in shares)	0	(4,244)	(81,112)	(95,658)
Forfeited (in shares)	0	(16,540)	(2,374)	(48,084)
Outstanding, end of period (in shares)	199,130	188,169	199,130	188,169
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 4.2	$ 4.1
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 41.22	$ 39.71	$ 39.75	$ 38.10
Granted (in dollars per share)	0	0	42.41	41.31
Vested (in dollars per share)	0	39.49	39.37	37.73
Forfeited (in dollars per share)	0	39.99	41.79	39.19
Outstanding, end of period (in dollars per share)	$ 41.22	$ 39.69	$ 41.22	$ 39.69
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	79,284	76,730	71,574	90,974
Granted (in shares)	0	0	27,123	26,469
Vested (in shares)	0	0	(18,691)	(29,042)
Forfeited (in shares)	0	(4,829)	(722)	(16,500)
Outstanding, end of period (in shares)	79,284	71,901	79,284	71,901
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 1.5	$ 1.5
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 50.28	$ 47.74	$ 47.67	$ 42.86
Granted (in dollars per share)	0	0	55.98	54.92
Vested (in dollars per share)	0	0	48.62	41.07
Forfeited (in dollars per share)	0	48.52	48.92	44.33
Outstanding, end of period (in dollars per share)	$ 50.28	$ 47.69	$ 50.28	$ 47.69
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	357,477	293,711	309,589	306,342
Granted (in shares)	0	0	108,499	105,860
Vested (in shares)	0	0	(62,778)	(71,807)
Increase above target (in shares)	4,277	0	10,278	0
Forfeited (in shares)	0	(19,316)	(3,834)	(66,000)
Outstanding, end of period (in shares)	361,754	274,395	361,754	274,395
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 4.6	$ 4.4
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.78	$ 39.91	$ 39.50	$ 38.42
Granted (in dollars per share)	0	0	42.41	41.31
Vested (in dollars per share)	0	0	48.07	37.68
Increase above target (in dollars per share)	38.02	0	37.77	0
Forfeited (in dollars per share)	0	38.60	43.53	37.31
Outstanding, end of period (in dollars per share)	$ 39.17	$ 40.00	$ 39.17	$ 40.00