Share-based compensation - Summary of changes in share based compensation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Restricted stock units
Shares
Outstanding, beginning of period (in shares)	233,448	193,939
Granted (in shares)	96,455	127,598
Vested (in shares)	(90,380)	(78,988)
Forfeited (in shares)	(31,178)	(6,358)
Outstanding, end of period (in shares)	208,345	236,191
Total weighted-average grant-date fair value of shares granted (in millions)	$ 4.0	$ 4.3
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.10	$ 40.89
Granted (in dollars per share)	41.29	33.98
Vested (in dollars per share)	37.58	38.51
Forfeited (in dollars per share)	38.78	42.20
Outstanding, end of period (in dollars per share)	$ 39.71	$ 37.91
LTIP linked to TRS
Shares
Outstanding, beginning of period (in shares)	90,974	89,222
Granted (in shares)	26,079	44,210
Vested (in shares)	(29,042)	(32,355)
Forfeited (in shares)	(11,671)	(1,024)
Outstanding, end of period (in shares)	76,340	100,053
Total weighted-average grant-date fair value of shares granted (in millions)	$ 1.4	$ 1.8
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 42.86	$ 42.10
Granted (in dollars per share)	54.92	41.12
Vested (in dollars per share)	41.07	38.20
Forfeited (in dollars per share)	42.60	45.89
Outstanding, end of period (in dollars per share)	$ 47.70	$ 42.89
LTIP awards linked to other performance conditions
Shares
Outstanding, beginning of period (in shares)	306,342	220,715
Granted (in shares)	104,300	176,844
Vested (in shares)	(71,807)	(43,155)
Increase above target (cancelled) (in shares)	0	(14,604)
Forfeited (in shares)	(46,684)	(4,098)
Outstanding, end of period (in shares)	292,151	335,702
Total weighted-average grant-date fair value of shares granted (in millions)	$ 4.3	$ 6.0
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 38.42	$ 41.03
Granted (in dollars per share)	41.29	33.98
Vested (in dollars per share)	37.68	34.12
Increase above target (in dollars per share)	0	43.90
Forfeited (in dollars per share)	36.77	44.36
Outstanding, end of period (in dollars per share)	$ 39.89	$ 38.04