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Postretirement Benefits - Narrative (Details) - Other Postretirement Benefits Plan - USD ($) $ in Millions | 12 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Mar. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |
Net actuarial gains | $ 8 | $ 11 |
Net prior service credits for post retirement plans | 11 | 14 |
Net actuarial gains for changes in benefit obligations | 3 | 14 |
Net prior service credits | 3 | $ 3 |
Expected amortization of actuarial gain from stockholders' equity in 2018 | 5 |  |
Amortization of actuarial gain from stockholders' equity | 6 |  |
Expected benefit payments in 2019 | 8 |  |
Expected benefit payments in 2020 | 7 |  |
Expected benefit payments in 2021 | 7 |  |
Expected benefit payments in 2022 | 7 |  |
Expected benefit payments in 2023 | 7 |  |
Other postretirement plan expected future benefit payments, 2024 through 2028 | 28 |  |
Expected contributions for postretirement benefit plans in 2018 | $ 8 |  |
Weighted-average discount rates used to estimate postretirement welfare benefit expenses | 3.83% | 3.68% | 3.59%
Weighted-average discount rates for actuarial present value of benefit obligations | 3.92% | 3.82% | 3.68%
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% |

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- Definition Defined Benefit Plan, Postretirement Welfare Benefit Plan, Actuarial Gain Loss Amortization Period Defined + References No definition available. + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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- Definition 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). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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