Nature of operations and summary of significant accounting policies (Cost reduction initiatives) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense		$ 691	$ 1,034
Predecessor
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense	$ 629			$ 2,879
One-time severance and termination benefits
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense		678	1,034
One-time severance and termination benefits | Predecessor
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense	608			2,772
Professional fees
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense		$ 13	$ 0
Professional fees | Predecessor
Restructuring Cost and Reserve [Line Items]
Cost reduction initiatives expense	$ 21			$ 107