Employee Benefit Plans - Weighted Average Discount Rate Used to Determine the Net Periodic Postretirement Benefit Costs (Details) - Other Postretirement Benefit Plan	4 Months Ended	8 Months Ended	12 Months Ended
	Dec. 31, 2017	Aug. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement benefit costs (as a percent)	3.61%
Accumulated postretirement benefit obligations (as a percent)	3.45%
Predecessor
Defined Benefit Plan Disclosure [Line Items]
Net periodic postretirement benefit costs (as a percent)		3.42%	3.50%
Accumulated postretirement benefit obligations (as a percent)		3.45%	3.61%