Share-based compensation - 2014 Plan RSUs (Details) - Restricted share units ("RSUs") - 2014 Plan $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Number of shares
Undistributed at beginning of year (in shares)	307,063
Granted (in shares)	408,350
Distributed (in shares)	(25,000)
Forfeited (in shares)	(2,275)
Undistributed at end of year (in shares)	688,138
Weighted average grant-date fair value
Undistributed at beginning of year (in dollars per share) | $ / shares	$ 9.11
Granted (in dollars per share) | $ / shares	5.83
Distributed (in dollars per share) | $ / shares	18.21
Forfeited (in dollars per share) | $ / shares	7.98
Undistributed at end of year (in dollars per share) | $ / shares	$ 6.84
Other disclosure
Total fair value of shares that vested (in dollars) | $	$ 2.4
Chief Executive Officer
Number of shares
Granted (in shares)	175,000
Other disclosure
Vesting period	2 years
Non-executive directors
Number of shares
Granted (in shares)	80,000
Other disclosure
Vesting period	1 year