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Benefit Plans Obligations and Funded Status (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2017
Change in benefit obligation |  |  |  |
Actuarial loss (gain) |  |  | $ (3) |
Change in plan assets |  |  |  |
Actual return on plan assets |  | $ 88 |  |
Amount related to experience loss |  | 59 |  |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Amount related to change in number of participants |  | 7 |  |
Amount related to change in payment elections |  |  | 46 |
Pension Benefits |  |  |  |
Change in benefit obligation |  |  |  |
Benefit obligation |  | 13,312 | 11,725 |
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation |  | 11,725 | 12,601 |
Service cost |  | 44 | 53 | $ 60
Interest cost |  | 499 | 473 | $ 464
Plan participants' contributions |  | 7 | 7 |
Change in Plan Assets due to Collections | [1] | 0 | 0 |
Collections | [1] | 0 | 0 |
Actuarial loss (gain) |  | 1,756 | (658) |
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment |  | 7 | 0 |
Net transfers from variable fund/401(k) plan |  | 1 | (26) |
Expenses paid |  | (6) | (6) |
Benefits paid |  | 721 | 719 |
Increase (Decrease) in Obligation, Other Postretirement Benefits |  |  | 110 |
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets |  | 7,980 | 8,003 |
Change in plan assets |  |  |  |
Fair value of net plan assets |  | 8,003 | 8,003 |
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets |  |  | 7,989 |
Actual return on plan assets |  | 389 | 454 |
Employer contributions |  | 307 | 304 |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Funded status |  | $ (5,332) | $ (3,722) |
Discount rate |  | 3.20% | 4.35% | 3.85%
Amount of defined benefit plan actuarial gain (loss) from discount rate change |  | $ (1,600) | $ (700) |
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption |  | (14) | (138) |
Other Post-retirement Benefits |  |  |  |
Change in benefit obligation |  |  |  |
Benefit obligation |  | 499 | 428 |
Postconfirmation, Other Postretirement Obligations |  | 428 | 494 |
Service cost |  | 11 | 14 | $ 18
Interest cost |  | 18 | 19 | $ 21
Plan participants' contributions |  | 0 | 0 |
Change in Plan Assets due to Collections | [1] | 22 | 25 |
Collections | [1] | 22 | 25 |
Actuarial loss (gain) |  | 78 | (46) |
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment |  | 0 | (17) |
Net transfers from variable fund/401(k) plan |  | 0 | 0 |
Expenses paid |  | 0 | 0 |
Benefits paid |  | 58 | 61 |
Increase (Decrease) in Obligation, Other Postretirement Benefits |  |  | 17 |
Change in plan assets |  |  |  |
Fair value of net plan assets |  | 0 | 0 |
Actual return on plan assets |  | 0 | 0 |
Employer contributions |  | 36 | 36 |
Defined Benefit Plan, Funded (Unfunded) Status of Plan [Abstract] |  |  |  |
Funded status |  | $ (499) | $ (428) |
Discount rate |  | 3.30% | 4.40% |
Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions |  | $ (24) | $ (8) |
Amount of change related to actual |  | 7 | 23 |
Amount of defined benefit plan actuarial gain (loss) from discount rate change |  | $ (71) | (28) |
Amount of defined benefit plan actuarial gain (loss) from change in mortality assumption |  |  | $ (6) |
Defined Benefit Plan, Assumptions Used in Calculation, Description |  | 0.033 | 0.044 | 0.0395
Amount of defined benefit plan actuarial gain (loss) from change in health care trend rate assumptions |  | $ 24 |  |
[1] | Collections include retiree contributions as well as provider discounts and rebates.

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- Definition Amount of gain (loss) from changes in actuarial assumptions related to discount rates which affects the defined benefit plan obligations. + References No definition available. + Details
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- Definition Amount of defined benefit plan actuarial gain (loss) from updated capita claims costs and retiree contributions + References No definition available. + Details
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- Definition The amount of contributions made by plan participants, federal reinsurance payments, and provider discounts and rebates. This item represents a periodic increase to the plan obligation and an increase to plan assets. + References No definition available. + Details
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- Definition Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(6) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964 + Details
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- Definition Amount of contribution received by defined benefit plan from employer which increases plan assets. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2709-114920 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920 + Details
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