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Commitments and Contingencies - Royalty Rates, Tax Assessment, Insurance and Other Matters (Details) $ in Millions | Jul. 13, 2015 a | Jun. 30, 2015 USD ($) | Jan. 22, 2015 Allotment_Parcel | Apr. 02, 2014 Allotment_Parcel | Apr. 01, 2014 USD ($) | Jan. 06, 2014 Allotment_Parcel | Sep. 30, 2012 Allotment_Parcel
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Continuous Highwall Mining [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public Utilities, Proposed Retroactive Surface Mining Royalty Rate |  | 12.50% |  |  |  |  |
Public Utilities, Current Surface Mining Royalty Rate applied between 2000 and 2003 |  | 8.00% |  |  |  |  |
Public Utilities, Estimated Underpaid Surface Mining Royalties under proposed rate change |  | $ 5.0 |  |  |  |  |
Public Utilities, PNM 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] |  |  |  |  |  |  |
Public Utilities, Assessed Coal Severance Surtax Penalty and Interest |  | $ 30.0 |  |  |  |  |
Public Service Company of New Mexico [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public Utilities, Ownership Percentage in Nuclear Reactor |  | 10.20% |  |  |  |  |
Public Utilities, Maximum Potential Assessment Per Incident |  | $ 38.9 |  |  |  |  |
Public Utilities, Annual Payment Limitation Related to Incident |  | 5.7 |  |  |  |  |
Public Utilities, Aggregate Amount of All Risk Insurance |  | 2,750.0 |  |  |  |  |
Public Utilities, Maximum Amount under Nuclear Electric Insurance Limited |  | 5.4 |  |  |  |  |
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] |  |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage |  | 13,400.0 |  |  |  |  |
Public Utilities, Liability Insurance Coverage Sublimit |  |  |  |  | $ 2,250.0 |  |
Public Service Company of New Mexico [Member] | Commercial Providers [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage |  | 375.0 |  |  |  |  |
Public Service Company of New Mexico [Member] | Industry Wide Retrospective Assessment Program [Member] | Nuclear Plant [Member] | Palo Verde Nuclear Generating Station [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public Utilities, Liability Insurance Coverage |  | $ 13,000.0 |  |  |  |  |
Public Service Company of New Mexico [Member] | NMTRD Coal Severance Tax [Member] | Four Corners [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Public Utilities, PNM Share Assessed Coal Severance Surtax Penalty and Interest to pass through FFPAC |  | 9.40% |  |  |  |  |
Public Service Company of New Mexico [Member] | Navajo Nation Allottee Matters [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Number of landowners claiming to be Navajo allottees | Allotment_Parcel |  |  |  |  |  |  | 43
Number of allotment parcels' appraisal requested for review | Allotment_Parcel |  |  |  |  |  | 58 |
Number of allotments where landowners are revoking rights of way renewal consents | Allotment_Parcel |  |  | 10 | 6 |  |  |
Public Service Company of New Mexico [Member] | Navajo Nation Allottee Matters [Member] | Subsequent Event [Member] |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |
Area of Land | a | 15.49 |  |  |  |  |  |

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