Share-based compensation - Summary of changes in share based compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	208,345	236,191	233,448	193,939
Granted (in shares)	2,008	4,894	98,463	132,492
Vested (in shares)	(1,034)	(292)	(91,414)	(79,280)
Forfeited (in shares)	(366)	(11,018)	(31,544)	(17,376)
Outstanding, end of period (in shares)	208,953	229,775	208,953	229,775
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.1	$ 0.2	$ 4.1	$ 4.6
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.71	$ 37.91	$ 38.10	$ 40.89
Granted (in dollars per share)	42.15	44.61	41.31	34.37
Vested (in dollars per share)	44.31	38.77	37.65	38.51
Forfeited (in dollars per share)	38.07	38.74	38.77	40.01
Outstanding, end of period (in dollars per share)	$ 39.71	$ 38.02	$ 39.71	$ 38.02
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	76,340	100,053	90,974	89,222
Granted (in shares)	390	1,533	26,469	45,743
Vested (in shares)	0	0	(29,042)	(32,355)
Forfeited (in shares)	0	(10,427)	(11,671)	(11,451)
Outstanding, end of period (in shares)	76,730	91,159	76,730	91,159
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.0	$ 0.1	$ 1.5	$ 1.9
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 47.70	$ 42.89	$ 42.86	$ 42.10
Granted (in dollars per share)	54.92	41.12	54.92	41.12
Vested (in dollars per share)	0	0	41.07	38.20
Forfeited (in dollars per share)	0	42.82	42.60	43.10
Outstanding, end of period (in dollars per share)	$ 47.74	$ 42.87	$ 47.74	$ 42.87
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	292,151	335,702	306,342	220,715
Granted (in shares)	1,560	6,133	105,860	182,977
Vested (in shares)	0	0	(71,807)	(43,155)
Increase above target (cancelled) (in shares)	0	15,881	0	1,277
Forfeited (in shares)	0	(41,711)	(46,684)	(45,809)
Outstanding, end of period (in shares)	293,711	316,005	293,711	316,005
Total weighted-average grant-date fair value of shares granted (at target performance levels) (in millions)	$ 0.1	$ 0.3	$ 4.4	$ 6.3
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 39.89	$ 38.04	$ 38.42	$ 41.03
Granted (in dollars per share)	42.37	44.49	41.31	34.33
Vested (in dollars per share)	0	0	37.68	34.12
Increase above target (in dollars per share)	0	42.92	0	31.71
Forfeited (in dollars per share)	0	38.27	36.77	38.82
Outstanding, end of period (in dollars per share)	$ 39.91	$ 38.38	$ 39.91	$ 38.38