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Benefit Plans Components of Net Periodic Benefit Cost (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Sep. 30, 2018 | Mar. 31, 2018 | Mar. 31, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Other Pension Plan |  |  |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |  |  |
Defined benefit plan contributions |  |  |  | $ 150,000,000 |  |  | $ 800,000,000
Pension Benefits |  |  |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |  |  |
Service cost | $ 13,000,000 | $ 13,000,000 |  | 27,000,000 | $ 30,000,000 |  |
Interest cost | 119,000,000 | 116,000,000 |  | 237,000,000 | 232,000,000 |  |
Expected return on plan assets | (119,000,000) | (114,000,000) |  | (239,000,000) | (228,000,000) |  |
Amortization of prior service cost | (24,000,000) | (24,000,000) |  | (49,000,000) | (49,000,000) |  |
Recognized net actuarial loss | 101,000,000 | 120,000,000 |  | 204,000,000 | 236,000,000 |  |
Total net periodic benefit cost as actuarially determined | 90,000,000 | 111,000,000 |  | 180,000,000 | 221,000,000 |  |
Amount capitalized due to actions of regulator | (13,000,000) | (34,000,000) |  | (27,000,000) | (68,000,000) |  |
Total net periodic benefit cost | 77,000,000 | 77,000,000 |  | 153,000,000 | 153,000,000 |  |
Other Post-retirement Benefits |  |  |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |  |  |
Defined benefit plan contributions |  |  |  | 9,000,000 | 17,000,000 |  |
Service cost | 3,000,000 | 5,000,000 |  | 7,000,000 | 9,000,000 |  |
Interest cost | 4,000,000 | 4,000,000 |  | 9,000,000 | 11,000,000 |  |
Expected return on plan assets | 0 | 0 |  | 0 | 0 |  |
Amortization of prior service cost | (5,000,000) | (10,000,000) |  | (11,000,000) | (11,000,000) |  |
Recognized net actuarial loss | 2,000,000 | 5,000,000 |  | 4,000,000 | 7,000,000 |  |
Total net periodic benefit cost as actuarially determined | 4,000,000 | 4,000,000 |  | 9,000,000 | 16,000,000 |  |
Amount capitalized due to actions of regulator | 0 | 0 |  | 0 | 0 |  |
Total net periodic benefit cost | $ 4,000,000 | $ 4,000,000 |  | $ 9,000,000 | $ 16,000,000 |  |
Scenario, Forecast | Other Pension Plan |  |  |  |  |  |  |
Retirement Plan Disclosure |  |  |  |  |  |  |
Defined benefit plan contributions |  |  | $ 150,000,000 |  |  | $ 300,000,000 |

X
- Definition Amount capitalized into regulatory assets due to actions of regulator related to pension benefits + References No definition available. + Details
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- Definition The total amount of net periodic benefit cost actuarilly determined for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. + References No definition available. + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive 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 (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit 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 (h)(5) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(ii) -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 715 -SubTopic 20 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit 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 (h)(3) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost recognized for passage of time related to defined benefit 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 (h)(2) -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 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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