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Segment Information (Narrative and Major Customers) (Details) (Sales [Member]) | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
First Choice [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations (as a percent) | 19.00% | 17.00% | 18.00%
Unaffiliated Customer of Texas-New Mexico Power Company One [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations (as a percent) | 17.00% | 19.00% | 19.00%
Unaffiliated Customer of Texas-New Mexico Power Company Two [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations (as a percent) | 10.00% | 12.00% | 13.00%
PNMR and PNM [Member] | Maximum [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Operating revenues from continuing operations (as a percent) | 10.00% |  |

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