Postretirement Benefit Plans (Future Benefit Payments) (Details) $ in Millions	12 Months Ended
	Mar. 31, 2017 USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	$ 858
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 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	79
Pension Benefits [Member] | 2022 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	84
Pension Benefits [Member] | 2023 through 2027 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	481
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	88
Other Benefits [Member] | 2018 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	6
Other Benefits [Member] | 2019 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	6
Other Benefits [Member] | 2020 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	7
Other Benefits [Member] | 2021 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	8
Other Benefits [Member] | 2022 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	9
Other Benefits [Member] | 2023 through 2027 [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments	$ 52