Share-based compensation - RSU activity (Details) - 2014 Plan $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2022 USD ($) $ / shares shares
Other disclosure
Vesting period (in years)	4 years
Non-executive directors
Other disclosure
Vesting period (in years)	1 year
Restricted share units ("RSUs")
Number of shares
Non-vested at beginning of period (in shares) | shares	710,617
Granted (in shares) | shares	1,562,133
Vested (in shares) | shares	(283,349)
Forfeited (in shares) | shares	(176,110)
Non-vested at end of period (in shares) | shares	1,813,291
Weighted average grant-date fair value
Non-vested at beginning of period (in dollars per share) | $ / shares	$ 38.89
Granted (in dollars per share) | $ / shares	15.91
Vested (in dollars per share) | $ / shares	39.47
Forfeited (in dollars per share) | $ / shares	23.76
Non-vested at end of period (in dollars per share) | $ / shares	$ 20.47
Other disclosure
Total weighted average grant date fair value of RSUs granted during the period (in millions) | $	$ 24.8
Restricted share units ("RSUs") | Non-executive directors
Other disclosure
Vesting period (in years)	1 year
Minimum | Restricted share units ("RSUs")
Other disclosure
Vesting period (in years)	1 year
Maximum | Restricted share units ("RSUs")
Other disclosure
Vesting period (in years)	3 years