Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 294	$ 252
Interest cost	984	941
Expected return on plan assets	(518)	(548)
Administrative expenses	2	2
Recognized actuarial (benefit) loss	561	446
Net periodic benefit cost	1,323	1,093
Other Benefits
Net Period Benefit Cost Assumptions [Line Items]
Service cost	17	20
Interest cost	48	50
Amortization of prior service cost	(695)	(1,086)
Recognized actuarial (benefit) loss	(252)	(285)
Net periodic benefit cost	$ (882)	$ (1,301)