Equity - Restricted stock (Details) - USD ($) $ / shares in Units, $ in Thousands	7 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other non-option information
Stock-based compensation expense		$ 3,861	$ 3,701	$ 127
System rental
Other non-option information
Stock-based compensation expense		6
System services
Other non-option information
Stock-based compensation expense		191
Transloading services
Other non-option information
Stock-based compensation expense		$ 3
Restricted stock
Number of Shares
Unvested, beginning (in shares)	648,676	1,218,265
Awarded (in shares)	584,477	88,664
Vested (in shares)		(644,387)
Forfeited (in shares)	(14,888)	(251,045)
Unvested, end (in shares)	1,218,265	411,497	1,218,265
Weighted Average Grant Date Fair Value
Unvested, beginning (in dollars per share)	$ 12.04	$ 12.61
Awarded (in dollars per share)	13.25	16.92
Vested (in dollars per share)		12.58
Forfeited (in dollars per share)	12.87	12.49
Unvested, end (in dollars per share)	$ 12.61	$ 13.67	$ 12.61
Other non-option information
Unrecognized compensation costs		$ 4,361
Expected period for recognizing compensation expense		1 year 10 months 13 days
Available for grant (in shares)		3,595,738
Restricted stock | First vesting period
Other non-option information
Unrecognized compensation costs (in shares)		213,636
Restricted stock | Second vesting period
Other non-option information
Unrecognized compensation costs (in shares)		179,790
Restricted stock | Third vesting period
Other non-option information
Unrecognized compensation costs (in shares)		18,071
Restricted stock | Salaries, benefits and payroll taxes
Other non-option information
Stock-based compensation expense		$ 3,661	$ 3,406
Restricted stock | Property, Plant and Equipment
Other non-option information
Stock-based compensation expense		1,040	$ 1,080
Restricted stock | System rental
Other non-option information
Stock-based compensation expense		6
Restricted stock | System services
Other non-option information
Stock-based compensation expense		191
Restricted stock | Transloading services
Other non-option information
Stock-based compensation expense		$ 3