Postretirement Benefits - Narrative (Details) - Other Postretirement Benefits Plan - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gains	$ 7	$ 8
Net prior service credits for post retirement plans	9	11
Net actuarial gains for changes in benefit obligations	1	3
Net prior service credits	2	$ 3
Expected amortization of actuarial gain from stockholders' equity in 2020	5
Amortization of actuarial gain from stockholders' equity	5
Expected benefit payments in 2020	7
Expected benefit payments in 2021	7
Expected benefit payments in 2022	7
Expected benefit payments in 2023	7
Expected benefit payments in 2024	6
Other postretirement plan expected future benefit payments, 2025 through 2029	26
Expected contributions for postretirement benefit plans in 2018	$ 7
Weighted-average discount rates used to estimate postretirement welfare benefit expenses	3.79%	3.83%	3.68%
Weighted-average discount rates for actuarial present value of benefit obligations	3.92%	3.92%	3.82%
Amortization period	3 years
Assumed health care cost trend in measuring accumulated postretirement benefit obligation for prescription drugs (as a percent)	3.00%	3.00%

Defined Benefit Plan, Postretirement Welfare Benefit Plan, Actuarial Gain Loss Amortization Period Defined

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

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Amount, before tax, after reclassification adjustment, of increase (decrease) in accumulated other comprehensive income from gain (loss) of defined benefit plan.

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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income from prior service cost (credit) of defined benefit plan.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position.

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Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in fourth fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.

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Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.

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Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

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Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

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