Quarterly report pursuant to Section 13 or 15(d)

Employee Benefit Plans - Components of Postretirement Benefit Cost and Pension Cost (Detail)

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Employee Benefit Plans - Components of Postretirement Benefit Cost and Pension Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Pension Plans, Defined Benefit [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 150 $ 150 $ 299 $ 300
Interest cost 355 363 711 725
Expected return on assets (745) (719) (1,489) (1,437)
Amortization of actuarial loss 254 338 507 676
Net pension cost and postretirement benefit cost 14 132 28 264
Other Postretirement Benefit Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 7 8 13 14
Amortization of actuarial loss 9 9 19 19
Net pension cost and postretirement benefit cost $ 16 $ 17 $ 32 $ 33