Share-based compensation - Changes in Share Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	22,172	89,869	65,628	189,024
Granted (in shares)	0	0	0	0
Vested (in shares)	0	0	(42,452)	(98,084)
Forfeited (in shares)	0	0	(1,004)	(1,071)
Outstanding, end of period (in shares)	22,172	89,869	22,172	89,869
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.41	$ 42.08	$ 42.09	$ 41.23
Granted (in dollars per share)	0	0	0	0
Vested (in dollars per share)	0	0	41.92	40.43
Forfeited (in dollars per share)	0	0	42.41	41.97
Outstanding, end of period (in dollars per share)	$ 42.41	$ 42.08	$ 42.41	$ 42.08
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	543,467	109,262	98,441	76,477
Granted (in shares)	0	0	462,313	62,152
Vested (in shares)	0	0	(17,287)	(28,577)
Forfeited (in shares)	0	0	0	(790)
Outstanding, end of period (in shares)	543,467	109,262	543,467	109,262
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 5.2	$ 1.1
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 13.50	$ 33.75	$ 31.36	$ 50.11
Granted (in dollars per share)	0	0	11.25	17.28
Vested (in dollars per share)	0	0	54.92	41.12
Forfeited (in dollars per share)	0	0	0	55.64
Outstanding, end of period (in dollars per share)	$ 13.50	$ 33.75	$ 13.50	$ 33.75
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	438,967	561,812	438,967	327,085
Granted (in shares)	0	0	0	362,963
Vested (in shares)	0	0	0	(113,118)
Increase above target/(cancelled) due to performance (in shares)	(57,007)	(49,603)	(57,007)	(61,556)
Forfeited (in shares)	0	0	0	(3,165)
Outstanding, end of period (in shares)	381,960	512,209	381,960	512,209
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.0	$ 0.0	$ 4.8
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 18.17	$ 23.25	$ 18.17	$ 39.44
Granted (in dollars per share)	0	0	0	13.09
Vested (in dollars per share)	0	0	0	34.93
Increase above target/(cancelled) due to performance (in dollars per share)	42.41	41.32	42.41	41.52
Forfeited (in dollars per share)	0	0	0	42.06
Outstanding, end of period (in dollars per share)	$ 14.55	$ 21.50	$ 14.55	$ 21.50