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Employee Benefit Plans - Weighted Average Actuarial Assumptions Used to Develop Projected Benefit Obligation (Detail) - Other Postretirement Benefit Plans [Member]
Mar. 31, 2018
Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Discount rate 3.63% 3.23%
Medical care cost trend rate 8.00% 8.00%