Schedule of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Plan and Supplemental Plan
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 254	$ 234	$ 506	$ 468
Interest cost	941	935	1,882	1,870
Expected return on plan assets	(548)	(530)	(1,097)	(1,060)
Administrative expenses	2		4
Amortization of prior service cost		9		18
Recognized actuarial (benefit) loss	446	567	892	1,134
Net periodic benefit cost	1,095	1,215	2,187	2,430
Other Benefits
Net Period Benefit Cost Assumptions [Line Items]
Service cost	20	75	40	150
Interest cost	50	211	100	421
Amortization of prior service cost	(1,086)	(510)	(2,172)	(1,021)
Recognized actuarial (benefit) loss	(285)	(245)	(570)	(489)
Net periodic benefit cost	$ (1,301)	$ (469)	$ (2,602)	$ (939)