Postretirement Benefit Plans (Future Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	$ 783
Pension Benefits [Member] | 2016 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	62
Pension Benefits [Member] | 2017 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	65
Pension Benefits [Member] | 2018 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	67
Pension Benefits [Member] | 2019 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	71
Pension Benefits [Member] | 2020 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	76
Pension Benefits [Member] | 2021 through 2025 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	442
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	84
Other Benefits [Member] | 2016 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	10
Other Benefits [Member] | 2017 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	9
Other Benefits [Member] | 2018 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	8
Other Benefits [Member] | 2019 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	7
Other Benefits [Member] | 2020 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	7
Other Benefits [Member] | 2021 through 2025 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	$ 43