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Asset Retirement Obligations | 6 Months Ended
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Mar. 31, 2018
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | During the six months ended March 31, 2018, TVA's total asset retirement obligation ("ARO") liability increased $85 million as a result of revisions in estimates and periodic accretion, partially offset by settlement projects that were conducted during this period. The revisions in estimate are primarily related to changes in strategy of asset retirements at certain TVA facilities. The nuclear and non-nuclear accretion expenses were deferred as regulatory assets. During the six months ended March 31, 2018, $72 million of the related non-nuclear regulatory assets were amortized into expense as these amounts were collected in rates. See Note 6. TVA maintains investment trusts to help fund its decommissioning obligations. See Note 14 — Investment Funds and Note 17 — Contingencies — Decommissioning Costs for a discussion of the trusts' objectives and the current balances of the trusts.
Asset Retirement Obligation Activity(1)
| Nuclear |  | Non-Nuclear |  | Total
Balance at September 30, 2017 | $ | 2,859 |  |  | $ | 1,445 |  |  | $ | 4,304 |
Settlements | — |  |  | (42 | ) |  | (42 | )
Revisions in estimate | — |  |  | 45 |  |  | 45 |
Additional obligations | — |  |  | 1 |  |  | 1 |
Accretion (recorded as regulatory asset) | 65 |  |  | 16 |  |  | 81 |
Balance at March 31, 2018 | $ | 2,924 |  |  | $ | 1,465 |  |  | $ | 4,389 |
Note (1) The current portion of ARO in the amount of $142 million and $128 million is included in Accounts payable and accrued liabilities at March 31, 2018, and September 30, 2017, respectively.

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- Definition The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671 + Details
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