Share-based compensation - Changes in Share Based Compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	90,903	202,133	189,024	182,528
Granted (in shares)	0	0	0	100,088
Vested (in shares)	(1,034)	(1,035)	(98,084)	(81,112)
Forfeited (in shares)	0	(1,968)	(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)	$ 42.11	$ 41.25	$ 41.23	$ 39.75
Granted (in dollars per share)	0	0	0	42.41
Vested (in dollars per share)	44.31	44.31	40.43	39.37
Forfeited (in dollars per share)	0	42.49	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)	109,643	80,006	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)	(381)	(722)	(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)	$ 33.82	$ 50.27	$ 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)	55.66	48.92	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)	574,744	355,310	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)	(11,407)	6,001	(11,953)	6,001
Forfeited (in shares)	(1,525)	(3,834)	(3,165)	(3,834)
Outstanding, end of period (in shares)	561,812	357,477	561,812	357,477
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)	$ 23.68	$ 38.87	$ 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)	42.37	36.08	42.32	36.08
Forfeited (in dollars per share)	42.07	43.53	42.06	43.53
Outstanding, end of period (in dollars per share)	$ 23.25	$ 38.78	$ 23.25	$ 38.78