Share-based compensation - 2014 Plan RSUs (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		9 Months Ended
	Jun. 30, 2018	Mar. 31, 2017	Sep. 30, 2018
Other disclosure
Vesting period	3 years
2014 Plan | Non-executive directors
Other disclosure
Vesting period			4 years
Restricted share units ("RSUs") | 2014 Plan
Number of shares
Non-vested at beginning of year (in shares)			683,663
Granted (in shares)			262,599
Vested (in shares)			(276,833)
Forfeited (in shares)			(189,708)
Non-vested at end of year (in shares)			479,721
Weighted average grant-date fair value
Non-vested at beginning of year (in dollars per share)			$ 6.38
Granted (in dollars per share)			23.61
Vested (in dollars per share)			5.99
Forfeited (in dollars per share)			8.00
Non-vested at end of year (in dollars per share)			$ 15.40
Other disclosure
Total weighted average grant date fair value of RSUs granted during the period (in millions)			$ 6.2
Restricted share units ("RSUs") | 2014 Plan | Chief Executive Officer
Other disclosure
Vesting period		2 years
Restricted share units ("RSUs") | 2014 Plan | Non-executive directors
Other disclosure
Vesting period		1 year
Restricted share units ("RSUs") | 2014 Plan | Directors and Officers
Number of shares
Granted (in shares)			133,808
Weighted average grant-date fair value
Granted (in dollars per share)			$ 3.3
Minimum | Restricted share units ("RSUs") | 2014 Plan
Other disclosure
Vesting period			1 year
Maximum | Restricted share units ("RSUs") | 2014 Plan
Other disclosure
Vesting period			3 years