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Pension and Other Postretirement Benefit Plans Narrative (Details) (USD $) | 12 Months Ended | 9 Months Ended | 12 Months Ended | 9 Months Ended | 12 Months Ended | 1 Months Ended
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Dec. 31, 2012 | Dec. 31, 2012 Pension Plan [Member] | Dec. 31, 2012 Other Postretirement Benefits [Member] | Dec. 31, 2012 Public Service Company of New Mexico [Member] Pension Plan [Member] | Dec. 31, 2011 Public Service Company of New Mexico [Member] Pension Plan [Member] | Sep. 30, 2012 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member] | Dec. 31, 2011 Public Service Company of New Mexico [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Texas-New Mexico Power Company [Member] Pension Plan [Member] | Dec. 31, 2011 Texas-New Mexico Power Company [Member] Pension Plan [Member] | Sep. 30, 2012 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member] | Dec. 31, 2011 Texas-New Mexico Power Company [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Equity Securities [Member] Pension Plan [Member] | Dec. 31, 2012 Equity Securities [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Debt Securities [Member] Pension Plan [Member] | Dec. 31, 2012 Debt Securities [Member] Other Postretirement Benefits [Member] | Dec. 31, 2012 Alternative Investments [Member] Pension Plan [Member] | Dec. 31, 2012 Developed Countries Outside of United States [Member] Equity Securities [Member] Pension Plan [Member] | Dec. 31, 2012 Minimum [Member] Pension Plan [Member] | Dec. 31, 2012 Maximum [Member] Pension Plan [Member] | Jan. 31, 2013 Subsequent Event [Member] Public Service Company of New Mexico [Member] Pension Plan [Member] | Jan. 31, 2013 Subsequent Event [Member] Texas-New Mexico Power Company [Member] Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Defined Benefit Plans, Gains and losses that lie outside the corridor, amortized (in years) | 5 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Benefit obligation, period increase (decrease) |  |  |  | $ 86,400,000 | $ 2,800,000 | $ 13,100,000 |  | $ (1,000,000) | $ 10,700,000 | $ (1,200,000) | $ 2,000,000 |  | $ (100,000) |  |  |  |  |  |  |  |  |  |
Defined Benefit Plan, Benefit Obligation, Period Increase (Decrease) Due to Change in Demographics |  |  |  | (8,000,000) | 1,600,000 | 8,100,000 |  | (1,700,000) | (800,000) | 1,700,000 | (800,000) |  | 600,000 |  |  |  |  |  |  |  |  |  |
Actual plan asset allocations (as a percent) |  |  |  | 11.00% |  |  | 16.30% |  | 11.20% |  |  | 16.10% |  |  |  |  |  |  |  |  |  |  |
Expected long-term return resulting from effect of one-percentage point decrease (as a percent) |  | 1.00% | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected Long-term return on assets decrease resulting in increase net periodic costs In next fiscal year |  |  |  | 5,500,000 |  |  | 600,000 |  | 600,000 |  |  | 100,000 |  |  |  |  |  |  |  |  |  |  |
Expected long-term return resulting from effect of one-percentage point increase (as a percent) |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Target plan asset allocations (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  | 31.00% | 70.00% | 53.00% | 30.00% | 16.00% | 8.00% |  |  |  |
Employer contributions |  |  |  | 77,700,000 | 41,511,000 |  | 3,529,000 | 2,873,000 | 5,300,000 | 1,151,000 |  | 342,000 | 351,000 |  |  |  |  |  |  |  |  | 60,000,000 | 1,000,000
Estimated future employer contributions from 2013-2017 |  |  |  | 49,100,000 |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average discount rate related to anticipated contributions (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.80% | 5.20% |  |
Early Retirement Portion Relating to Regulated Operations |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated future employer contributions in next fiscal year |  |  |  |  |  |  | $ 3,500,000 |  |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |  |  |

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- Definition Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets Decrease Resulting in Increase Net Periodic Costs In Next Fiscal Year + References No definition available. + Details
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- Definition Defined Benefit Plan, Estimated Future Employer Contributions in Year Two Through Year Four + References No definition available. + Details
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- Definition Net increase or decrease in the plan benefit obligation during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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