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Assumptions Used in Accounting for Pension Plan (Detail) - Pension
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]      
Discount rate - benefit obligations [1] 8.00% 8.10% 8.60%
Discount rate - net periodic benefit cost [1] 8.00% 8.10% 8.60%
Rate of increase in compensation levels of covered employees 8.00% 8.00% 8.00%
Rate of return on plan assets 7.00% 7.00% 8.00%
[1] Weighted average assumptions used to determine both benefit obligations and net periodic benefit cost.