Employee Benefit Plans - Components of the Net Periodic Pension and Postretirement Benefit Cost (Details) - USD ($) $ in Thousands	4 Months Ended	8 Months Ended	12 Months Ended
	Dec. 31, 2017	Aug. 31, 2017	Dec. 31, 2016
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 140
Interest cost	1,574
Expected return on assets	(2,767)
Amortization of prior service cost	0
Amortization of actuarial gain (loss)	0
Net periodic pension plans benefit	(1,053)
Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	12
Interest cost	12
Amortization of actuarial gain (loss)	0
Net periodic pension plans benefit	$ 24
Predecessor | Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 260	$ 377
Interest cost		3,194	5,182
Expected return on assets		(5,425)	(8,139)
Amortization of prior service cost		133	200
Amortization of actuarial gain (loss)		631	1,832
Net periodic pension plans benefit		(1,207)	(548)
Predecessor | Other Postretirement Benefit Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		22	71
Interest cost		33	65
Amortization of actuarial gain (loss)		(193)	(209)
Net periodic pension plans benefit		$ (138)	$ (73)