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Schedule of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 1 Months Ended 2 Months Ended 3 Months Ended 4 Months Ended 6 Months Ended
Jul. 31, 2017
Sep. 30, 2017
Sep. 30, 2016
Jul. 31, 2017
Sep. 30, 2016
Pension Benefits          
Net Period Benefit Cost Assumptions [Line Items]          
Service cost   $ 203      
Interest cost   624      
Expected return on plan assets   (332)      
Administrative expenses   2      
Net periodic benefit cost   497      
Pension Benefits | Predecessor          
Net Period Benefit Cost Assumptions [Line Items]          
Service cost $ 99   $ 254 $ 393 $ 506
Interest cost 328   941 1,313 1,882
Expected return on plan assets (173)   (548) (691) (1,097)
Administrative expenses 1   2 3 4
Recognized actuarial (benefit) loss 187   446 748 892
Net periodic benefit cost 442   1,095 1,766 2,187
Other Benefits          
Net Period Benefit Cost Assumptions [Line Items]          
Service cost   12      
Interest cost   30      
Net periodic benefit cost   $ 42      
Other Benefits | Predecessor          
Net Period Benefit Cost Assumptions [Line Items]          
Service cost 6   20 23 40
Interest cost 16   50 64 100
Amortization of prior service cost (232)   (1,086) (927) (2,172)
Recognized actuarial (benefit) loss (83)   (285) (335) (570)
Net periodic benefit cost $ (293)   $ (1,301) $ (1,175) $ (2,602)