Postretirement Benefits - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Expected amortization of actuarial loss from stockholders' equity in 2017	$ 15
Expected benefit payments in 2017	59
Expected benefit payments in 2018	174
Expected benefit payments in 2019	110
Expected benefit payments in 2020	66
Expected benefit payments in 2021	65
Other postretirement plan expected future benefit payments, 2022 through 2026	327
Expected contributions for postretirement benefit plans in 2016	15
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	4	$ 1
Net prior service credits for post retirement plans	16	1
Net actuarial loss for changes in benefit obligations	3	$ 9
Net prior service credits	16
Expected amortization of actuarial loss from stockholders' equity in 2017	1
Amortization of actuarial gain from stockholders' equity	1
Expected benefit payments in 2017	11
Expected benefit payments in 2018	9
Expected benefit payments in 2019	9
Expected benefit payments in 2020	8
Expected benefit payments in 2021	8
Other postretirement plan expected future benefit payments, 2022 through 2026	34
Expected contributions for postretirement benefit plans in 2016	$ 11
Weighted-average discount rates used to estimate postretirement welfare benefit expenses	3.59%	4.07%	3.84%
Weighted-average discount rates for actuarial present value of benefit obligations	3.68%	3.61%	4.08%
Amortization period	3 years
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for prescription drugs (as a percent)	6.50%	6.75%
Other Postretirement Benefit Plan, Defined Benefit | 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)	7.00%	7.25%
Pension and other postretirement benefit plans, assumptions used calculating postretirement benefit obligation, age threshold	65 years
Other Postretirement Benefit Plan, Defined Benefit | 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)	6.50%	6.75%
Pension and other postretirement benefit plans, assumptions used calculating postretirement benefit obligation, age threshold	65 years
Other Postretirement Benefit Plan, Defined Benefit | Dental
Defined Benefit Plan Disclosure [Line Items]
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for ages (as a percent)	5.00%	5.00%