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Asset Retirement Obligations | 12 Months Ended
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Dec. 31, 2020
Asset Retirement Obligation Disclosure [Abstract] |
Asset Retirement Obligations | Asset Retirement Obligations AROs are recorded based on studies to estimate the amount and timing of future ARO expenditures and reflect underlying assumptions, such as discount rates, estimates of the future costs for decommissioning, and the timing of the removal activities to be performed. Approximately 76% of PNM’s total ARO liabilities are related to nuclear decommissioning of PVNGS. PNM is responsible for all decommissioning obligations related to its entire interest in PVNGS, including portions under lease both during and after termination of the leases. Studies of the decommissioning costs of PVNGS, SJGS, Four Corners, and other facilities are performed periodically and revisions to the ARO liabilities are recorded. Changes in the assumptions underlying the calculations may also require revisions to the estimated AROs when identified. A reconciliation of the ARO liabilities is as follows:
| PNMR |  | PNM |  | TNMP
| (In thousands)
Liability at December 31, 2017 | $ | 146,679 |  |  | $ | 145,707 |  |  | $ | 793 |
Liabilities incurred | — |  |  | — |  |  | — |
Liabilities settled | (192) |  |  | — |  |  | — |
Accretion expense | 11,482 |  |  | 11,402 |  |  | 67 |
Revisions to estimated cash flows | 705 |  |  | 705 |  |  | — |
Liability at December 31, 2018 | 158,674 |  |  | 157,814 |  |  | 860 |
Liabilities incurred | — |  |  | — |  |  | — |
Liabilities settled | (987) |  |  | (935) |  |  | (52) |
Accretion expense | 12,635 |  |  | 12,562 |  |  | 73 |
Revisions to estimated cash flows(1) | 11,640 |  |  | 11,640 |  |  | — |
Liability at December 31, 2019 | 181,962 |  |  | 181,081 |  |  | 881 |
Liabilities incurred | — |  |  | — |  |  | — |
Liabilities settled | (1,444) |  |  | (1,192) |  |  | (252) |
Accretion expense | 11,310 |  |  | 11,235 |  |  | 75 |
Revisions to estimated cash flows(2) | (8,407) |  |  | (8,407) |  |  | — |
Liability at December 31, 2020 | $ | 183,421 |  |  | $ | 182,717 |  |  | $ | 704 |
(1) Reflects the impacts of an updated SJGS decommissioning study that assumes PNM will retire its share of SJGS in 2022. (2) Reflects a decrease of $9.2 million related to an updated PVNGS decommissioning study and an increase of $0.8 million related to an updated Four Corners decommissioning study.

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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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