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Commitments and Contingencies - SJGS Matters (Details) | Sep. 30, 2015 MW | Aug. 13, 2015 MW | May. 19, 2015 MW | Jan. 07, 2015 MW | Oct. 01, 2014 MW | Jul. 31, 2014 MW | Jun. 26, 2014 USD ($) MW | Dec. 20, 2013 MW
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Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Public utilities, unsubscribed ownership (in megawatts) |  |  |  | 65 |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Jointly owned utility plan, proposed proportionate ownership share |  |  |  |  |  |  | 64.50% |
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 1 and 4 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Jointly owned utility plan, proposed proportionate ownership share |  |  |  |  |  |  | 58.70% |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 3 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Percentage of ownership held by exiting owners | 50.00% |  |  |  |  |  |  |
Public utilities, ownership percentage | 50.00% |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Percentage of ownership held by exiting owners | 38.80% |  |  |  |  |  |  |
Public utilities, ownership percentage | 38.50% |  |  |  |  |  |  |
Public utilities, additional ownership to be obtained (in megawatts) | 132 | 65 | 132 |  | 132 | 132 | 132 | 78
Public utilities, costs to obtain additional ownership | $ |  |  |  |  |  |  | $ 0 |
PNMR Development [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Public utilities, potential acquisition of ownership (in megawatts) | 65 |  | 65 |  |  |  |  |

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