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Regulatory and Rate Matters - New Mexico General Rate Cases (Details) $ in Thousands |  |  |  |  |  |  |  |  |  |  |  |  | 9 Months Ended | 12 Months Ended | 30 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jan. 31, 2018 USD ($) | Jan. 16, 2018 USD ($) | Jan. 10, 2018 USD ($) | Dec. 31, 2017 USD ($) | May 23, 2017 USD ($) | Dec. 07, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 28, 2016 USD ($) MW | Aug. 04, 2016 USD ($) MW | Jan. 15, 2016 USD ($) lease MW | Aug. 27, 2015 USD ($) | Jan. 15, 2015 | Sep. 30, 2016 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2018 USD ($) | Oct. 31, 2017 USD ($) | May 12, 2017 signatory | Jul. 01, 2016 USD ($) | May 31, 2016 MW
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred federal income tax |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 7,716 | $ 119,182 | $ 60,892 |  |  |  |  |
Pre-tax regulatory disallowance for capital costs |  |  |  |  |  |  |  |  |  |  |  |  |  | 36,800 |  |  | $ 58,000 |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  |  |  |  |  |  |  | 65,598 | 27,036 | 15,011 |  |  |  |  |
Increase (decrease) in non-fuel revenue requirement | $ 10,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of non-fuel revenue requirement change implemented | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 99,200 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  |  | 10.125% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of additional signatories | signatory |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 13 |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  | 21 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance |  |  |  | $ 27,900 |  |  |  |  |  |  |  |  |  |  | 27,900 |  |  |  |  |  |
Public Service Company of New Mexico |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred federal income tax |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,661 | 73,308 | 49,123 |  |  |  |  |
Regulatory disallowances and restructuring costs |  |  |  |  |  |  |  |  |  |  |  |  |  | 66,339 | $ 27,036 | $ 15,011 |  |  |  |  |
Public Service Company of New Mexico | NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  | $ (4,400) | $ (9,100) |  | $ 62,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  | 9.575% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial rate increase (decrease) amount |  |  |  |  | $ 32,300 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease), duplicative amount |  | $ 4,700 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net book value |  |  |  |  |  |  |  |  |  |  |  |  |  | 50,000 |  |  | 50,000 |  |  |  |
Public Service Company of New Mexico | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 | Installation costs including construction management, gross receipts taxes, AFUDC, and other PNM costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Portion of costs for SNCRs and BDT equipment |  |  |  |  |  |  |  |  | $ 52,300 |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 2 Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 800 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  | 73,300 |  |  | 73,300 |  |  |  |
Net book value of capital improvements |  |  |  |  |  |  |  |  |  |  |  |  |  | 38,000 |  |  | 38,000 |  |  |  |
Public Service Company of New Mexico | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated annual property tax expense |  |  |  |  |  |  |  |  |  | $ 1,500 |  |  |  |  |  |  |  |  |  |  |
Estimated annual rent expense |  |  |  |  |  |  |  |  | $ 18,100 |  |  |  |  |  |  |  |  |  |  |  |
Number of leases under which lease term was extended | lease |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |
Lease term extension period |  |  |  |  |  |  |  |  |  |  |  | 8 years |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 2014 Electric Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Future test year period |  |  |  |  |  | 12 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Redefined future test year as period that begins a period of time following the filing of a rate case application (up to) |  |  |  |  |  | 13 months |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  |  |  |  |  | $ 123,500 |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  |  |  |  |  |  |  | 10.50% |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) in fuel-related costs |  |  |  |  |  |  |  |  |  |  |  |  | $ 42,900 |  |  |  |  |  |  |  |
Change in requested rate increase (decrease) in non-fuel related revenues |  |  |  |  |  |  |  |  |  |  |  |  |  | 200 |  |  |  |  |  |  |
Requested rate increase (decrease) in non-fuel related revenue |  |  |  |  |  |  |  |  | 121,500 |  |  |  |  | 121,500 |  |  |  |  |  |  |
Proposed increase (decrease) in non-fuel revenue |  |  |  |  |  |  |  |  | $ 41,300 |  |  |  |  |  |  |  |  |  |  |  |
Proposed return on equity |  |  |  |  |  |  |  |  | 9.575% |  |  |  |  |  |  |  |  |  |  |  |
Approved rate increase (decrease) |  |  |  |  |  |  |  | $ 61,200 |  |  |  |  |  |  |  |  |  |  |  |  |
Estimated period of time for New Mexico Supreme Court Appeal decision |  |  |  |  |  |  | 15 months |  |  |  |  |  |  |  | 7 months |  |  |  |  |  |
Minimum amount of loss of capital cost recovery |  |  |  |  |  |  | 15 months |  |  |  |  |  |  |  |  |  |  |  |  |  |
Estimate of possible loss |  |  |  |  |  |  |  |  |  |  |  |  |  | 146,100 |  |  | $ 146,100 |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | San Juan Generating Station | Refined Coal |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed credit to retail customers |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  |  |  |  |  |  |  |  |
Deferred federal income tax |  |  |  |  |  |  |  |  | $ 2,100 |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Clean Air Act, Balanced Draft Technology | San Juan Generating Station Units 1 and 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of equipment costs included in rate base |  |  |  |  |  |  |  |  | 40,000 |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 2 Leases |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed disallowance of recovery of purchase price of assets |  |  |  |  |  |  |  |  | 163,300 |  |  |  |  |  |  |  |  |  |  |  |
Approved lease acquisition costs |  |  |  |  |  |  |  | 83,700 |  |  |  |  |  |  |  |  |  |  |  |  |
Disallowed leasehold improvements |  |  |  |  |  |  |  | 43,800 |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance for capital costs |  |  |  | $ 3,100 |  |  | $ 6,800 |  |  |  |  |  |  | 4,000 |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Palo Verde Nuclear Generating Station, Unit 1 Leases, extended |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Approved recovery of assumed operating and maintenance expense savings annually related to BDT |  |  |  |  |  |  |  | $ 300 |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Alvarado square |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed disallowance of recovery of assets and deferred charges from retail customers |  |  |  |  |  |  |  |  | $ 4,500 |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4,500 |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Non-Fuel Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  |  |  |  |  | $ 121,700 |  |  |  |  |  |  |  |  |  |
Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 90,100 |  | $ 148,100 |
Regulatory disallowances and restructuring costs |  |  | $ 47,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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