Postretirement Benefit Plans (Pension Plan Obligations) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 1,865	$ 1,868	$ 1,863
Accumulated benefit obligation	1,711	1,692
Pension plans with accumulated benefit obligations in excess of plan assets, accumulated benefit obligation	1,581	1,567
Pension plans with accumulated benefit obligations in excess of plan assets, fair value of plan assets	1,049	1,039
Other Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plans with projected benefit obligations in excess of plan assets, projected benefit obligation	1,769	1,868
Pension plans with projected benefit obligations in excess of plan assets, fair value of plan assets	1,103	1,177
Pension plans with accumulated benefit obligations in excess of plan assets, projected benefit obligation	96	0
Pension plans with accumulated benefit obligations in excess of plan assets, fair value of plan assets	$ 97	$ 0