Schedule of Net Periodic Benefit Cost for Postretirement Health Care and Life Insurance Plan (Detail) (Other Benefits, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Other Benefits
Net Period Benefit Cost Assumptions [Line Items]
Service cost	$ 475	$ 554	$ 581
Interest cost	1,235	1,379	1,458
Amortization of prior service cost	(2,032)	(2,032)	(2,032)
Recognized actuarial loss		(4)	(20)
Net periodic (benefit) cost	$ (322)	$ (103)	$ (13)