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Postretirement Benefit Plans (Target and Actual Allocation Percentages) (Details)
12 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Equity [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Target Allocation Percentage 15-55%  
Target allocation ranges, minimum 15.00%  
Target allocation ranges, maximum 53.00%  
Allocation in aggregate 32.00% 32.00%
Fixed Income [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Target Allocation Percentage 45-77%  
Target allocation ranges, minimum 47.00%  
Target allocation ranges, maximum 68.00%  
Allocation in aggregate 49.00% 62.00%
Real Estate [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Target Allocation Percentage 0-15%  
Target allocation ranges, minimum 0.00%  
Target allocation ranges, maximum 15.00%  
Allocation in aggregate 3.00% 2.00%
Other [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Target Allocation Percentage 0-16%  
Target allocation ranges, minimum 0.00%  
Target allocation ranges, maximum 16.00%  
Allocation in aggregate 17.00% 5.00%