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Commitments and Contingencies - Nuclear Spent Fuel and Waste Disposal (Details) - Public Service Company of New Mexico - Palo Verde Nuclear Generating Station - Nuclear spent fuel and waste disposal - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
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Loss Contingencies [Line Items] |  |
Estimate of possible loss | $ 57.7 |
Other deferred credits |  |
Loss Contingencies [Line Items] |  |
Liability for interim storage costs | $ 12.4 | $ 12.3

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