Stock incentive plans - Restricted stock unit awards (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2018
RSUs
Stock incentive plans
Vesting period		3 years
Number of Shares
Non-vested at beginning of year (in shares)		3,324,000
Granted (in shares)		978,000
Vested (in shares)		(1,112,938)
Canceled/forfeited (in shares)		(70,000)
Non vested at end of year (in shares)		3,119,000
Weighted Average Grant Date Fair Value
Non-vested at beginning of year (in dollars per share)		$ 7.35
Granted (in dollars per share)		13.73
Vested (in dollars per share)		8.99
Canceled/forfeited (in dollars per share)		14.98
Non vested at end of year (in dollars per share)		$ 8.60
Other Disclosures
Shares of common stock issued resulting from vesting		702,447
Total unrecognized stock-based compensation expense		$ 9,314
Weighted average period over which unrecognized compensation expense is expected to be recognized		2 years 7 months 6 days
Time-based restricted stock unit awards | Executive And Non Executive Member | Maximum
Stock incentive plans
Vesting period		3 years
Time-based restricted stock unit awards | Non-employee directors and existing members
Stock incentive plans
Vesting period		1 year
Time-based restricted stock unit awards | Non-employee directors and new members
Stock incentive plans
Vesting percentage when the individual completes 12 months of continuous service		33.30%
Vesting period		3 years
Performance-based restricted stock unit awards
Stock incentive plans
Vesting period		3 years
Number of Shares
Granted (in shares)	584,000
Weighted Average Grant Date Fair Value
Granted (in dollars per share)	$ 6.13
Performance-based restricted stock unit awards | Minimum
Stock incentive plans
Performance objective threshold (as a percent)		50.00%
Performance-based restricted stock unit awards | Maximum
Stock incentive plans
Performance objective threshold (as a percent)		150.00%