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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations In 2019, APS received updated decommissioning estimates for the Navajo Plant closure in December 2019, which resulted in a decrease to the ARO in the amount of $8 million (see Note 4 for additional information). In addition, APS received a new decommissioning study for Palo Verde. This resulted in a decrease to the ARO in the amount of $89 million, a decrease in plant in service of $80 million and a reduction in the regulatory liability of $9 million. In 2018, APS recognized an ARO for the removal of hazardous waste containing solar panels at all of our utility scale solar plants, which resulted in an increase to the ARO in the amount of $14 million. In addition, due to the sale of 4CA assets to NTEC in 2018 (see Note 11 for more information on 4CA matters) there was a decrease to the ARO of $9 million. APS recognized an ARO of $7 million for rooftop solar removals in accordance with the obligations included in the customer contracts, which requires APS to remove the panels at the end of the contract life and includes the costs for the disposal of hazardous materials in accordance with environmental regulations. Finally, APS has other ARO adjustments resulting in a net decrease of $1 million. The following table shows the change in our asset retirement obligations for 2019 and 2018 (dollars in thousands):
| 2019 |  | 2018
Asset retirement obligations at the beginning of year | $ | 726,545 |  |  | $ | 679,529 |
Changes attributable to: |  |  |  |  |
Accretion expense | 39,726 |  |  | 36,876 |
Settlements | (12,591 | ) |  | (9,726 | )
Estimated cash flow revisions | (96,462 | ) |  | 2,002 |
Newly incurred or acquired obligations | — |  |  | 17,864 |
Asset retirement obligations at the end of year | $ | 657,218 |  |  | $ | 726,545 |
In accordance with regulatory accounting, APS accrues removal costs for its regulated utility assets, even if there is no legal obligation for removal. See detail of regulatory liabilities in Note 4.

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -URI http://asc.fasb.org/subtopic&trid=2175671 + Details
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