Postretirement Benefits - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of actuarial gain from stockholders' equity in 2018	$ (11)
Expected benefit payments in 2018	73
Expected benefit payments in 2019	206
Expected benefit payments in 2020	61
Expected benefit payments in 2021	62
Expected benefit payments in 2022	64
Other postretirement plan expected future benefit payments, 2022 through 2026	307
Expected contributions for postretirement benefit plans in 2018	30
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain (loss)	11	$ (4)
Net prior service credits for post retirement plans	14	16
Net actuarial (loss) gain for changes in benefit obligations	14	(3)
Net prior service credits	3	$ 16
Expected amortization of actuarial gain from stockholders' equity in 2018	4
Amortization of actuarial gain from stockholders' equity	1
Expected benefit payments in 2018	8
Expected benefit payments in 2019	7
Expected benefit payments in 2020	7
Expected benefit payments in 2021	7
Expected benefit payments in 2022	7
Other postretirement plan expected future benefit payments, 2022 through 2026	30
Expected contributions for postretirement benefit plans in 2018	$ 8
Weighted-average discount rates used to estimate postretirement welfare benefit expenses	3.68%	3.59%	4.07%
Weighted-average discount rates for actuarial present value of benefit obligations	3.82%	3.68%	3.61%
Amortization period	3 years
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for prescription drugs (as a percent)	3.00%	6.50%
Other Postretirement Benefit Plan [Member] | Medical | Pre-65
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for ages (as a percent)	3.00%	7.00%
Pension and other postretirement benefit plans, assumptions used calculating postretirement benefit obligation, age threshold	65 years
Other Postretirement Benefit Plan [Member] | Medical | Post-65
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for ages (as a percent)	3.00%	6.50%
Pension and other postretirement benefit plans, assumptions used calculating postretirement benefit obligation, age threshold	65 years
Other Postretirement Benefit Plan [Member] | Dental
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for ages (as a percent)	3.00%	5.00%