Schedule of Net Periodic Benefit Cost for Postretirement Health Care and Life Insurance Plan (Detail) - Other Benefits - USD ($) $ in Thousands	4 Months Ended	5 Months Ended	12 Months Ended
	Jul. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Mar. 31, 2017
Net Period Benefit Cost Assumptions [Line Items]
Service cost		$ 29	$ 61
Interest cost		75	117
Amortization of prior service cost			(299)
Recognized actuarial (gain)			42
Net curtailment gain			4,005
Net periodic pension cost		$ 104	$ (4,084)
Predecessor
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 23			$ 81
Interest cost	64			201
Amortization of prior service cost	(927)			(4,346)
Recognized actuarial (gain)	(335)			(1,138)
Net periodic pension cost	$ (1,175)			$ (5,202)