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Retirement Benefits - Target Investment Allocations (Details)
12 Months Ended
Mar. 31, 2019
Rate
Mar. 31, 2018
Rate
Equity securities    
Defined Benefit Plan Disclosure [Line Items]    
Investment policy, minimum allocation 20.00%  
Investment policy, maximum allocation 30.00%  
Target Allocation 28.00%  
Actual Allocation 26.00% 28.00%
Debt securities (including cash and cash equivalents)    
Defined Benefit Plan Disclosure [Line Items]    
Investment policy, minimum allocation 55.00%  
Investment policy, maximum allocation 80.00%  
Target Allocation 64.00%  
Actual Allocation 64.00% 65.00%
Other    
Defined Benefit Plan Disclosure [Line Items]    
Investment policy, minimum allocation 0.00%  
Investment policy, maximum allocation 10.00%  
Target Allocation 9.00%  
Actual Allocation 10.00% 7.00%