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Summary of Business and Significant Accounting Policies (Details) (USD $) | 12 Months Ended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2011 Maximum [Member] | Dec. 31, 2011 Public Service Company of New Mexico [Member] | Dec. 31, 2010 Public Service Company of New Mexico [Member] | Dec. 31, 2009 Public Service Company of New Mexico [Member] | Dec. 31, 2011 Texas-New Mexico Power Company [Member] | Dec. 31, 2010 Texas-New Mexico Power Company [Member] | Dec. 31, 2009 Texas-New Mexico Power Company [Member] | Sep. 23, 2011 Optim Energy [Member] | Sep. 22, 2011 Optim Energy [Member] | Dec. 31, 2010 Optim Energy [Member] | Jan. 04, 2012 Sale of Ownership Interst [Member] Optim Energy [Member] | Dec. 31, 2011 Coal [Member] | Dec. 31, 2010 Coal [Member] | Dec. 31, 2011 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] Operating_Leases | Dec. 31, 1986 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] Operating_Leases | Dec. 31, 1985 Palo Verde Nuclear Generating Station [Member] Public Service Company of New Mexico [Member] Operating_Leases | Dec. 31, 2004 EIP Transmission Line [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2003 EIP Transmission Line [Member] Public Service Company of New Mexico [Member] | Dec. 31, 1985 EIP Transmission Line [Member] Public Service Company of New Mexico [Member] Operating_Leases | Dec. 31, 2011 10.3% Lessor Notes [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2011 10.15% Lessor Notes [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2011 10.25% EIP Lessor Note Maturing in 2012 [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2010 10.25% EIP Lessor Note Maturing in 2012 [Member] Public Service Company of New Mexico [Member] | Dec. 31, 2011 10.25% EIP Lessor Note Maturing in 2012 [Member] EIP Transmission Line [Member] Public Service Company of New Mexico [Member]
Accounting Policies Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity method investment,ownership percentage (as a percent) |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents Maturity Term |  |  |  | 3 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allowance for funds used during construction, capitalized interest |  |  |  |  | $ 1,500,000 | $ 4,600,000 | $ 8,000,000 | $ 1,200,000 | $ 200,000 | $ 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt, weighted average interest rate (as a percent) | 6.60% | 6.70% | 6.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest costs incurred, capitalized | 500,000 | 800,000 | 700,000 |  | 200,000 | 400,000 | 200,000 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Prepaid coal, current |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14,600,000 | 30,900,000 |  |  |  |  |  |  |  |  |  |  |
Number of operating leases (in ones) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8 | 11 | 11 |  |  | 2 |  |  |  |  |
Notes receivable, stated percentage rate (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.30% | 10.15% |  |  | 10.25%
Ownership percentage acquired (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 40.00% | 60.00% |  |  |  |  |  |
Other investments | 12,207,000 | 18,791,000 |  |  | 2,900,000 | 5,068,000 |  | 271,000 | 282,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,200,000 | 4,600,000 |
Impairment losses on securities held in the NDT |  |  |  |  | 12,500,000 | 5,000,000 | 8,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage, sold (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Equity Method Investments |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost method investment, ownership percentage (as a percent) |  |  |  |  |  |  |  |  |  |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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