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Summary of Business and Significant Accounting Policies (Inventories/Depreciation and Amortization) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 54,231 | 52,479 |
Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 17,175 | 17,115 |
Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 37,056 | 35,364 |
Public Service Company of New Mexico [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 51,521 | 49,454 |
Public Service Company of New Mexico [Member] | Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 17,175 | 17,115 |
Public Service Company of New Mexico [Member] | Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 34,346 | 32,339 |
Public Service Company of New Mexico [Member] | Electric plant [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used (as a percent) | 2.24% | 2.24% | 2.31%
Public Service Company of New Mexico [Member] | Common, intangible, and general plant [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Depreciation average rates used (as a percent) | 6.03% | 5.63% | 4.29%
Texas-New Mexico Power Company [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 2,710 | 2,787 |
Depreciation average rates used (as a percent) | 3.41% | 3.39% | 3.44%
Texas-New Mexico Power Company [Member] | Coal [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 0 | 0 |
Texas-New Mexico Power Company [Member] | Materials and supplies [Member] |  |  |
Public Utilities, Inventory [Line Items] |  |  |
Inventory, Net | 2,710 | 2,787 |

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- Definition Carrying amount (lower of cost or market) as of the balance sheet date of inventories less all valuation and other allowances. Excludes noncurrent inventory balances (expected to remain on hand past one year or one operating cycle, if longer). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 330 -SubTopic 10 -Section 35 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6386567&loc=d3e3927-108312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6361293&loc=d3e6676-107765 + Details
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- Definition Discloses the overall composite depreciation rate for pubic utility plants in service for each year an income statement is presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 10.C) -URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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