Share-based compensation - PSU activity (Details) - Performance share units ("PSUs") - 2014 Plan $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Number of shares
Non-vested at beginning of year (in shares) | shares	377,169
Restricted stock granted (in shares) | shares	132,362
Forfeited (in shares) | shares	(23,064)
Non-vested at end of year (in shares) | shares	486,467
PSUs awarded but not yet earned | shares	96,389
Total non-vested and discretionary PSUs | shares	582,856
Weighted average grant-date fair value
Non-vested at beginning of year (in dollars per share) | $ / shares	$ 16.73
Granted (in dollars per share) | $ / shares	31.71
Forfeited (in dollars per share) | $ / shares	5.76
Non-vested at end of year (in dollars per share) | $ / shares	17.80
PSUs awarded but not yet earned (in dollars per share) | $ / shares	59.65
Total non-vested and discretionary PSUs (in dollars per share) | $ / shares	$ 24.72
Other disclosure
Total weighted average grant date fair value of RSUs granted during the period (in millions) | $	$ 9.9