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Commitments and Contingencies - Nuclear Spent Fuel and Waste Disposal (Details) - PNM - Nuclear spent fuel and waste disposal - Palo Verde Nuclear Generating Station - USD ($) | May 16, 2014 | Mar. 31, 2018 | Dec. 31, 2017
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Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Estimate of possible loss |  | $ 57,700,000 |
Revised annual fee, nuclear waste disposal | $ 0 |  |
Other deferred credits |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |
Loss contingency accrual |  | $ 12,200,000 | $ 12,300,000

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