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Commitments and Contingencies - The Clean Air Act (Details) |  |  | 3 Months Ended | 12 Months Ended | 35 Months Ended |  |  |  |  |  |  |  |  |  |  |
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Dec. 16, 2015 USD ($) MWh MW | Mar. 02, 2015 lb / MMBTU T | Mar. 31, 2016 USD ($) MW | Mar. 31, 2015 USD ($) | Dec. 31, 2015 USD ($) | Jan. 31, 2016 USD ($) | Feb. 09, 2016 | Jan. 26, 2016 state | Oct. 01, 2015 parts_per_billion | Sep. 30, 2015 parts_per_billion | Aug. 11, 2015 T | Jul. 31, 2015 MW | Dec. 30, 2013 lbsofnox / mmbtu | Aug. 06, 2012 compliance_alternative | Jan. 31, 2010 parts_per_billion | Jul. 31, 2005 T | Dec. 31, 1999 state
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | $ 774,000 | $ 215,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service, held for future use, and to be abandoned |  |  | 6,676,612,000 |  | $ 6,307,261,000 |  |  |  |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  | $ 2,246,332,000 |  | 2,058,772,000 |  |  |  |  |  |  |  |  |  |  |  |
Clean Air Act related to Regional Haze [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states to address regional haze (in states) | state |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50
Potential to emit tons per year of visibility impairing pollution (in tons, more than) | T |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 250 |
Clean Power Plan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of potential extension to meet environmental targets |  |  | 2 years |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states that filed a petition against the Clean Power Plan | state |  |  |  |  |  |  |  | 29 |  |  |  |  |  |  |  |  |
Proposed percentage reduction in CO2 emissions, stayed by the USSC |  |  |  |  |  |  | 32.00% |  |  |  |  |  |  |  |  |  |
Period of time currently remaining to submit initial plans to meet environmental targets |  |  | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time currently remaining to submit initial plans to meet environmental targets, including potential extension |  |  | 2 years 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  | $ 774,000 | $ 215,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service, held for future use, and to be abandoned |  |  | 5,185,298,000 |  | 4,833,303,000 |  |  |  |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  | 1,744,062,000 |  | $ 1,569,549,000 |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station Units 2 and 3 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Plant in service, held for future use, and to be abandoned |  |  | 471,300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated depreciation and amortization |  |  | 197,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net book value |  |  | $ 274,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station And Four Corners [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed government standard emission limit (in ozone parts per million) | parts_per_billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60 |  |
Public Service Company of New Mexico [Member] | San Juan Generating Station And Four Corners [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed government standard emission limit (in ozone parts per million) | parts_per_billion |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 70 |  |
Government standard emission limit (in ozone parts per million) | parts_per_billion |  |  |  |  |  |  |  |  | 70 | 75 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated costs to remain unrecovered | $ 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Units 1 and 4 [Member] | Installation Costs Including Construction Management, Gross Receipts Taxes, AFUDC, and Other PNM Costs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total costs for equipment |  |  |  |  |  | $ 76,600,000 |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Units 2 and 3 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current ownership interest (in megawatts) | MW | 418 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Time period to recover retired units' remaining net book value | 20 years |  |  |  | 20 years |  |  |  |  |  |  |  |  |  |  |  |
Recovery percentage of estimated undepreciated value at 12/31/17 | 50.00% |  | 50.00% |  | 50.00% |  |  |  |  |  |  |  |  |  |  |  |
Estimated undepreciated value at 12/31/17 |  |  |  |  | $ 255,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | 127,600,000 |  |  |  |  |  |  |  |  |  |  |  |
Expense reflected in regulatory disallowances and restructuring costs |  |  | $ 800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accumulated plant write-off, disallowance |  |  | 128,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net carrying value of utility plant |  |  | 145,900,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Units 2 and 3 [Member] | Increase in coal mine decommissioning liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  | $ 165,700,000 |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 65 |  |  |  |  |  |  |  |  |  |  | 132 |  |  |  |  |
Estimated rate base value | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Coal-fired generation (in megawatts) | MW | 197 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatt hours of Renewable Energy Certificates to be acquired and retired (in megawatt hours) | MWh | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual cost of Renewable Energy Credits | $ 7,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated undepreciated value at 12/31/17 | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, SNCR Hearing Examiner Recommended Denial [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional ownership to be obtained (in megawatts) | MW | 132 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, Post-2022 Coal Supply future rate case [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time agreed to by all parties to settle rate case | 6 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act Related to Post Combustion Controls [Member] | Four Corners [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of compliance alternatives | compliance_alternative |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |
Plant requirement to meet NOx emissions limit (in pounds of NOx per MMBTU) | lbsofnox / mmbtu |  |  |  |  |  |  |  |  |  |  |  |  | 0.015 |  |  |  |
Plant requirement to meet opacity limit |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |
Rule imposes opacity limitation on certain fugitive dust emissions from coal and material handling operations |  |  |  |  |  |  |  |  |  |  |  |  | 20.00% |  |  |  |
Loss contingency, estimate of possible loss |  |  | $ 91,400,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act Related to Post Combustion Controls [Member] | Four Corners Units 4 and 5 (Coal) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  | 13.00% |  |  |  |
Public Service Company of New Mexico [Member] | National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time to act on settlement |  | 16 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Emissions tons of SO2 per year | T |  | 16,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Emissions tons of SO2 per year | T |  | 2,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public utilities, 1-hour SO2 emissions rate (in pounds per MMBTU) | lb / MMBTU |  | 0.45 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period of time from state designation, to provide implementation plans |  |  | 36 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | National Ambient Air Quality Standards, 2015 EPA Legal Settlement [Member] [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Emissions tons of SO2 per year | T |  |  |  |  |  |  |  |  |  |  | 2,000 |  |  |  |  |  |
PNMR and PNM [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts | MW |  |  | 134 |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNMR and PNM [Member] | Clean Air Act, SNCR [Member] | Palo Verde Nuclear Generating Station Unit 3 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of megawatts | MW | 134 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
PNMR Development [Member] | Clean Air Act, SNCR [Member] | San Juan Generating Station Unit 4 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense for cost, obligation to reimburse |  |  | $ 600,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Potential acquisition of ownership (in megawatts) | MW |  |  |  |  |  |  |  |  |  |  |  | 65 |  |  |  |  |

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