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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% |

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- Definition Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation + References No definition available. + Details
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- Definition Pension and Other Postretirement Benefit Plans, Assumptions Used Calculating Postretirement Benefit Obligation, Age Threshold + References No definition available. + Details
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- Definition The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 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=65897934&loc=d3e1928-114920 + Details
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- Definition Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Prior Service Cost -URI http://asc.fasb.org/extlink&oid=49177896 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=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 + Details
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- Definition The amount of gains or losses recognized in net periodic benefit cost. + 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 (h)(4) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount before tax, after reclassification adjustments, of actuarial gain (loss) that has not been recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount before tax, after reclassification adjustments, of prior service cost (credit) that has not been recognized in the income statement. + 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 (i) -URI http://asc.fasb.org/extlink&oid=65897934&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 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580 + Details
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- Definition The interest rate used to adjust for the time value of money for the plan. + 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 (k)(1) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition 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. + References No definition available. + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the five fiscal years after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + 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 (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + 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 (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + 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 (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + 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 (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of benefits from a defined benefit plan expected to be paid in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + 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 (f) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Health Care Cost Trend Rate -URI http://asc.fasb.org/extlink&oid=6514820 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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