Assumed Health Care Cost Trend Rates (Detail)	12 Months Ended
	Mar. 31, 2016
Pre Sixty Five Coverage
Schedule of Pension Expected Future Benefit Payments [Line Items]
Accumulated postretirement benefit obligation	7.75%
Net periodic postretirement benefit obligation	7.90%
Ultimate health care cost trend	4.54%
Ultimate year health care cost trend rate is achieved	2038
Net periodic postretirement benefit obligation March 31, 2016	7.75%
Post Sixty Five Coverage
Schedule of Pension Expected Future Benefit Payments [Line Items]
Accumulated postretirement benefit obligation	6.90%
Net periodic postretirement benefit obligation	6.90%
Ultimate health care cost trend	4.50%
Ultimate year health care cost trend rate is achieved	2029
Net periodic postretirement benefit obligation March 31, 2016	6.90%