Schedule of Net Periodic Benefit Cost for Postretirement Health Care and Life Insurance Plan (Detail) - Other Benefits - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 81	$ 212	$ 273
Interest cost	201	584	904
Amortization of prior service cost	(4,346)	(2,996)	(2,032)
Recognized actuarial (gain)	(1,138)	(1,040)	(1,299)
Net periodic pension cost	$ (5,202)	$ (3,240)	$ (2,154)