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Assumed Health Care Cost Trends Rates (Detail)
12 Months Ended
Mar. 31, 2014
Pre Sixty Five Coverage
 
Schedule of Pension Expected Future Benefit Payments [Line Items]  
Accumulated postretirement benefit obligation 8.20%
Net periodic postretirement benefit obligation 8.70%
Ultimate health care cost trend 4.50%
Ultimate year health care cost trend rate is achieved 2029
Net periodic postretirement benefit obligation for March 31, 2015 8.20%
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 for March 31, 2015 6.90%