Postretirement Benefit Plans (Pension Plan Obligations) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Pension plans with accumulated benefit obligations in excess of plan assets:
Accumulated benefit obligation	$ 1,563	$ 1,507
Fair value of plan assets	1,093	1,136
Defined Benefit Plan, fair value plan assets [Domain]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Benefit Obligation	1,129	1,163
Pension plans with accumulated benefit obligations in excess of plan assets:
Defined benefit plan, plans with benefit obligations less than plan assets	104	0
Defined Benefit Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	1,863	1,672	1,581
Accumulated benefit obligation	1,689	1,527
Projected Benefit Obligation [Domain]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plans with Benefit Obligations in Excess of Plan Assets, Aggregate Benefit Obligation	1,760	1,672
Pension plans with accumulated benefit obligations in excess of plan assets:
Defined benefit plan, plans with benefit obligations less than plan assets	$ 103	$ 0