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Regulatory and Rate Matters - New Mexico General Rate Cases (Details) $ in Millions |  |  |  |  |  |  |  |  | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 07, 2016 USD ($) | Sep. 30, 2016 | Sep. 28, 2016 USD ($) MW | Aug. 04, 2016 USD ($) MW | Jan. 15, 2016 USD ($) lease MW | Aug. 27, 2015 USD ($) | Jan. 15, 2015 | Dec. 11, 2014 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2016 USD ($) | May 31, 2016 MW
NMPRC [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) | $ 99.2 |  |  |  |  |  |  |  |  |  |
Requested return on equity | 10.125% |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Clean Air Act, Balanced Draft Technology [Member] | San Juan Generating Station Units 1 and 4 [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net book value |  |  |  |  |  |  |  |  |  | $ 50.0 |
Public Service Company of New Mexico [Member] | Clean Air Act, Balanced Draft Technology [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, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Portion of costs for SNCRs and BDT equipment |  |  |  | $ 52.3 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of megawatts purchased (in megawatts) | MW |  |  |  |  | 64.1 |  |  |  |  |  | 64.1
Number of leases under which assets were purchased | lease |  |  |  |  | 3 |  |  |  |  |  |
Estimated annual property tax expense |  |  |  | $ 0.8 |  |  |  |  |  |  |
Number of leases under which lease term was extended | lease |  |  |  |  | 1 |  |  |  |  |  |
Lease term extension period |  |  |  |  | 8 years |  |  |  |  |  |
Number of megawatts | MW |  |  | 114.6 | 114.6 |  |  |  |  |  |  |
Net book value |  |  |  |  |  |  |  |  |  | 76.9 |
Net book value of capital improvements |  |  |  |  |  |  |  |  |  | 39.9 |
Public Service Company of New Mexico [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Estimated annual property tax expense |  |  |  |  | $ 1.5 |  |  |  |  |  |
Estimated annual rent expense |  |  |  | $ 18.1 |  |  |  |  |  |  |
Number of leases under which lease term was extended | lease |  |  |  |  | 4 |  |  |  |  |  |
Lease term extension period |  |  |  |  |  |  | 8 years |  |  |  |
Public Service Company of New Mexico [Member] | 2014 Electric Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  |  | $ 107.4 |  |  |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 123.5 |  |  |  |  |
Requested return on equity |  |  |  |  |  | 10.50% |  |  |  |  |
Requested rate increase (decrease) in fuel-related costs |  |  |  |  |  |  |  |  | $ (42.9) |  |
Change in requested rate increase (decrease) in non-fuel related revenues |  |  |  |  |  |  |  |  |  | (0.2) |
Requested rate increase (decrease) in non-fuel related revenue |  |  |  | 121.5 |  |  |  |  |  | 121.5 |
Proposed increase (decrease) in non-fuel revenue |  |  |  | $ 41.3 |  |  |  |  |  |  |
Proposed return on equity |  |  |  | 9.575% |  |  |  |  |  |  |
Approved rate increase (decrease) |  |  | $ 61.2 |  |  |  |  |  |  |  |
Estimated period of time for New Mexico Supreme Court Appeal decision |  | 15 months |  |  |  |  |  |  |  |  |
Minimum amount of loss of capital cost recovery |  | 15 months |  |  |  |  |  |  |  |  |
Increase (decrease) of income tax expense |  |  |  |  |  |  |  |  |  | (2.1) |
Estimate of possible loss |  |  |  |  |  |  |  |  |  | 155.6 |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | San Juan Generating Station [Member] | Refined Coal [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proposed credit to retail customers |  |  |  | 100.00% |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Clean Air Act, Balanced Draft Technology [Member] | San Juan Generating Station Units 1 and 4 [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amount of equipment costs included in rate base |  |  |  | $ 40.0 |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 2 Leases [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proposed disallowance of recovery of purchase price of assets |  |  |  | 163.3 |  |  |  |  |  |  |
Approved lease acquisition costs |  |  | 83.7 |  |  |  |  |  |  |  |
Disallowed leasehold improvements |  |  | 43.8 |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance for capital costs |  |  |  |  |  |  |  |  |  | 6.8 |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Approved recovery of assumed operating and maintenance expense savings annually related to BDT |  |  | $ 0.3 |  |  |  |  |  |  |  |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Alvarado square [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proposed disallowance of recovery of assets and deferred charges from retail customers |  |  |  | $ 4.5 |  |  |  |  |  |  |
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges |  |  |  |  |  |  |  |  |  | $ 4.5 |
Public Service Company of New Mexico [Member] | 2015 Electric Rate Case [Member] | Non-Fuel Energy [Member] |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 121.7 |  |  |  |  |

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