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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) | Mar. 31, 2016 USD ($) | Dec. 01, 2015 Allotment_Parcel | Jul. 13, 2015 a | Jan. 22, 2015 Allotment_Parcel | Apr. 02, 2014 Allotment_Parcel | Jan. 06, 2014 Allotment_Parcel | Aug. 31, 2013 | May. 23, 2013 USD ($) | Sep. 30, 2012 landowner
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Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Ownership percentage in nuclear reactor | 10.20% |  |  |  |  |  |  |  |
Maximum potential assessment per incident | $ 38,900,000 |  |  |  |  |  |  |  |
Annual payment limitation related to incident | 5,800,000 |  |  |  |  |  |  |  |
Aggregate amount of all risk insurance | 2,750,000,000.00 |  |  |  |  |  |  |  |
Maximum amount under Nuclear Electric Insurance Limited | 5,400,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Liability insurance coverage | 13,500,000,000.0 |  |  |  |  |  |  |  |
Liability insurance coverage sublimit | 2,250,000,000.00 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Commercial Providers [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Liability insurance coverage | 375,000,000 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Industry Wide Retrospective Assessment Program [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Liability insurance coverage | $ 13,100,000,000.0 |  |  |  |  |  |  |  |
Continuous Highwall Mining [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Proposed retroactive surface mining royalty rate |  |  |  |  |  |  | 12.50% |  |
Surface mining royalty rate applied | 8.00% |  |  |  |  |  |  |  |
Estimated underpaid surface mining royalties under proposed rate change | $ 5,000,000 |  |  |  |  |  |  |  |
PNM's share estimated underpaid surface mining royalties under proposed rate change | 46.30% |  |  |  |  |  |  |  |
NMTRD Coal Severance Tax [Member] | Four Corners [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Assessed coal severance surtax, penalty and interest |  |  |  |  |  |  |  | $ 30,000,000 |
Share of settlement in addition to amounts previously paid | $ 100,000 |  |  |  |  |  |  |  |
Navajo Nation Allottee Matters [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |
Number of landowners claiming to be Navajo allottees (in landowners) | landowner |  |  |  |  |  |  |  |  | 43
Number of allotment parcels' appraisal requested for review (in allotment parcels) | Allotment_Parcel |  |  |  |  |  | 58 |  |  |
Number of allotments where landowners are revoking rights of way renewal consents (in allotment parcels) | Allotment_Parcel |  |  |  | 10 | 6 |  |  |  |
Area of land (in acres) | a |  |  | 15.49 |  |  |  |  |  |
Number of allotment parcels at issue that are not to be condemned | Allotment_Parcel |  | 2 |  |  |  |  |  |  |
Number of allotment parcels at issue | Allotment_Parcel |  | 5 |  |  |  |  |  |  |

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