Postretirement Benefit Plans (Actuarial Assumptions and Sensitivity Analysis) (Details)	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Weighted average assumptions used to determine net periodic benefit cost, Expected return on plan assets	5.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average assumptions used to determine benefit obligations, Discount rate	3.20%	3.30%	3.10%
Weighted average assumptions used to determine benefit obligations, Average compensation growth	3.10%	3.10%	3.10%
Weighted average assumptions used to determine net periodic benefit cost, Discount rate	3.30%	3.10%	4.00%
Weighted average assumptions used to determine net periodic benefit cost, Average compensation growth	3.10%	3.10%	3.10%
Weighted average assumptions used to determine net periodic benefit cost, Expected return on plan assets	5.40%	5.60%	6.10%
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Weighted average assumptions used to determine benefit obligations, Discount rate	4.10%	4.00%	3.60%
Weighted average assumptions used to determine benefit obligations, Average compensation growth	3.50%	3.50%	3.50%
Weighted average assumptions used to determine net periodic benefit cost, Discount rate	4.00%	3.60%	4.10%
Weighted average assumptions used to determine net periodic benefit cost, Average compensation growth	3.50%	3.50%	3.50%
Weighted average assumptions used to determine net periodic benefit cost, Expected return on plan assets	0.00%	0.00%	0.00%