Postretirement Benefit Plans (Pension Plan Assets) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 1,200	$ 1,177
Investments measured at net asset value	1,002	993
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	144	140
Investments measured at net asset value	0	0
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	54	44
Investments measured at net asset value	0	0
Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Investments measured at net asset value	0	0
Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Equity [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
Equity [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0	0
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	190	175
Fixed Income [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	136	131
Fixed Income [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	54	44
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	0	0
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	0	0
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	9
Cash and Cash Equivalents [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	8	9
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	0	0
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	0	0
Other [Member] | Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 0	$ 0