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Summary of Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2012
Accounting Policies [Abstract] |
Schedule of Inventory, Current [Table Text Block] | Inventories consisted of the following at December 31:
| PNMR |  | PNM |  | TNMP
| 2012 |  | 2011 |  | 2012 |  | 2011 |  | 2012 |  | 2011
| (In thousands)
Coal | $ | 19,231 |  |  | $ | 17,175 |  |  | $ | 19,231 |  |  | $ | 17,175 |  |  | $ | — |  |  | $ | — |
Materials and supplies | 40,412 |  |  | 37,056 |  |  | 37,559 |  |  | 34,346 |  |  | 2,853 |  |  | 2,710 |
| $ | 59,643 |  |  | $ | 54,231 |  |  | $ | 56,790 |  |  | $ | 51,521 |  |  | $ | 2,853 |  |  | $ | 2,710 |
Schedule of Average Rates Used Allocated Between Depreciation Expense and Construction Expense Projects Based on Use of Equipment [Table Text Block] | Average rates used are as follows:
| Year ended December 31
| 2012 |  | 2011 |  | 2010
PNM |  |  |  |  |
Electric plant | 2.25 | % |  | 2.24 | % |  | 2.24 | %
Common, intangible, and general plant | 5.35 | % |  | 6.03 | % |  | 5.63 | %
TNMP | 3.56 | % |  | 3.41 | % |  | 3.39 | %

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- Definition Schedule of Average Rates Used Allocated Between Depreciation Expense and Construction Expense Projects Based on Use of Equipment [Table Text Block] + References No definition available. + Details
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- Definition Tabular disclosure of the carrying amount as of the balance sheet date of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 2 -Paragraph 6 -Subparagraph a,b,c -Article 5 + Details
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