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
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 23	$ 29	$ 61
Interest cost	64	75	117
Amortization of prior service cost	(927)		(299)
Recognized actuarial (gain)	(335)		42
Net curtailment gain			(4,005)
Net periodic pension cost	$ (1,175)	$ 104	$ (4,084)