Share-based compensation - LTIP Linked to TRS (Details) - LTIP linked to TRS - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Shares
Outstanding, beginning of period (in shares)	89,222	96,402	65,578
Granted (in shares)	46,024	24,630	35,215
Vested (issued or unissued and cancelled) (in shares)	(32,355)	(29,409)	0
Forfeited (in shares)	(11,917)	(2,401)	(4,391)
Outstanding, end of period (in shares)	90,974	89,222	96,402
Weighted-Average Grant-Date Fair Value Per Share
Outstanding, beginning of period (in dollars per share)	$ 42.10	$ 39.62	$ 38.81
Granted (in dollars per share)	41.12	48.62	41.07
Vested (issued or unissued and cancelled) (in dollars per share)	38.20	39.51	0
Forfeited (in dollars per share)	43.07	41.22	39.19
Outstanding, end of period (in dollars per share)	$ 42.86	$ 42.10	$ 39.62
Total weighted-average grant-date fair value of shares granted (in millions)	$ 1.9	$ 1.2	$ 1.4