Share-based compensation - PSU activity (Details) - Performance share units ("PSUs") $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) $ / shares shares
Other disclosure
Vesting period (in years)	3 years
2014 Plan
Number of shares
Non-vested at beginning of year (in shares) | shares	479,422
Restricted stock granted (in shares) | shares	91,003
Forfeited (in shares) | shares	(354,105)
Non-vested at end of year (in shares) | shares	216,320
Weighted average grant-date fair value
Non-vested at beginning of year (in dollars per share) | $ / shares	$ 21.17
Granted (in dollars per share) | $ / shares	57.56
Forfeited (in dollars per share) | $ / shares	17.44
Non-vested at end of year (in dollars per share) | $ / shares	$ 42.58
Other disclosure
Total weighted average grant date fair value of RSUs granted during the period (in millions) | $	$ 5.2