Postretirement Benefit Plans (Pension Plan Assets) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 1,233	$ 1,163
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	228	54
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	1,005	1,109
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	446	453
Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	85	48
Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	361	405
Equity [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Fixed Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	743	665
Fixed Income [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	135	0
Fixed Income [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	608	665
Fixed Income [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	15	15
Real Estate [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Real Estate [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	15	15
Real Estate [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	8	6
Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	8	6
Cash and Cash Equivalents [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Cash and Cash Equivalents [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	21	24
Other [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Other [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	21	24
Other [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 0	$ 0