Share-based compensation - RSU activity (Details) - 2014 Plan - $ / shares	1 Months Ended	6 Months Ended
	Mar. 31, 2017	Jun. 30, 2020
Non-executive directors
Other disclosure
Vesting period (in years)		4 years
Restricted share units ("RSUs")
Number of shares
Non-vested at beginning of year (in shares)		370,830
Restricted stock granted (in shares)		244,449
Vested (in shares)		(171,401)
Forfeited (in shares)		(14,562)
Non-vested at end of year (in shares)		429,316
Weighted average grant-date fair value
Non-vested at beginning of year (in dollars per share)		$ 28.62
Granted (in dollars per share)		52.81
Vested (in dollars per share)		19.36
Forfeited (in dollars per share)		45.12
Non-vested at end of year (in dollars per share)		45.53
Restricted share units ("RSUs") | Non-executive directors
Other disclosure
Vesting period (in years)	1 year
Restricted share units ("RSUs") | Directors and Officers
Weighted average grant-date fair value
Granted (in dollars per share)		$ 12,900,000
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