Share-based compensation - Changes in Share Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	89,869	199,130	189,024	182,528
Granted (in shares)	0	0	0	100,088
Vested (in shares)	0	0	(98,084)	(81,112)
Forfeited (in shares)	0	0	(1,071)	(2,374)
Outstanding, end of period (in shares)	89,869	199,130	89,869	199,130
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 0.0	$ 4.2
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.08	$ 41.22	$ 41.23	$ 39.75
Granted (in dollars per share)	0	0	0	42.41
Vested (in dollars per share)	0	0	40.43	39.37
Forfeited (in dollars per share)	0	0	41.97	41.79
Outstanding, end of period (in dollars per share)	$ 42.08	$ 41.22	$ 42.08	$ 41.22
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	109,262	79,284	76,477	71,574
Granted (in shares)	0	0	62,152	27,123
Vested (in shares)	0	0	(28,577)	(18,691)
Forfeited (in shares)	0	0	(790)	(722)
Outstanding, end of period (in shares)	109,262	79,284	109,262	79,284
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 1.1	$ 1.5
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 33.75	$ 50.28	$ 50.11	$ 47.67
Granted (in dollars per share)	0	0	17.28	55.98
Vested (in dollars per share)	0	0	41.12	48.62
Forfeited (in dollars per share)	0	0	55.64	48.92
Outstanding, end of period (in dollars per share)	$ 33.75	$ 50.28	$ 33.75	$ 50.28
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	561,812	357,477	327,085	309,589
Granted (in shares)	0	0	362,963	108,499
Vested (in shares)	0	0	(113,118)	(62,778)
Increase above target (cancelled) (in shares)	(49,603)	4,277	(61,556)	10,278
Forfeited (in shares)	0	0	(3,165)	(3,834)
Outstanding, end of period (in shares)	512,209	361,754	512,209	361,754
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 4.8	$ 4.6
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 23.25	$ 38.78	$ 39.44	$ 39.50
Granted (in dollars per share)	0	0	13.09	42.41
Vested (in dollars per share)	0	0	34.93	48.07
Increase above target (cancelled) (in dollars per share)	41.32	38.02	41.52	37.77
Forfeited (in dollars per share)	0	0	42.06	43.53
Outstanding, end of period (in dollars per share)	$ 21.50	$ 39.17	$ 21.50	$ 39.17