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Regulatory and Rate Matters - New Mexico General Rate Cases (Details) $ in Millions |  |  |  |  |  |  |  |  |  |  |  |  |  | 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 ($) | Jan. 15, 2015 | Dec. 11, 2014 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2018 USD ($) | Oct. 31, 2017 USD ($) | Jul. 01, 2016 USD ($) | May 31, 2016 MW
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in non-fuel revenue requirement | $ 10.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of non-fuel revenue requirement change implemented | 50.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  | $ 99.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  |  | 10.125% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance |  |  |  | $ 27.9 |  |  |  |  |  |  |  |  |  |  |  | $ 27.9 |  |  |  |
Public Service Company of New Mexico | NMPRC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  | $ 62.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested return on equity |  |  |  |  | 9.575% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proposed term for providing benefits to customers related to reduction in state corporate tax |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Initial rate increase (decrease) amount |  |  |  |  | $ 32.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Service Company of New Mexico | NMPRC | Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  | $ (4.4) | $ (9.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease), duplicative amount |  | $ 4.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  | 49.4 |  |  |  |  |  |  |  |  |  |  |  | 49.4 |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 52.3 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 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 |  |  |  | 75.3 |  |  |  |  |  |  |  |  |  |  |  | 75.3 |  |  |  |
Net book value of capital improvements |  |  |  | 39.1 |  |  |  |  |  |  |  |  |  |  |  | 39.1 |  |  |  |
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 |  |  |  |  |  |  |  |  |  | $ 1.5 |  |  |  |  |  |  |  |  |  |
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 | 2014 Electric Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  |  |  |  |  |  |  | $ 107.4 |  |  |  |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | 7 months |  |  |  |
Minimum amount of loss of capital cost recovery |  |  |  |  |  |  | 15 months |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) of income tax expense |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (2.1) |  |  |  |
Estimate of possible loss |  |  |  | 151.1 |  |  |  |  |  |  |  |  |  |  |  | 151.1 |  |  |  |
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% |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 40.0 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | 163.3 |  |  |  |  |  |  |  |  |  |  |
Approved lease acquisition costs |  |  |  |  |  |  |  | 83.7 |  |  |  |  |  |  |  |  |  |  |  |
Disallowed leasehold improvements |  |  |  |  |  |  |  | 43.8 |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance for capital costs |  |  |  | $ 3.1 |  |  | $ 6.8 |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  | $ 0.3 |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  | $ 4.5 |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance of costs recorded as regulatory assets and deferred charges |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.5 |  |  |  |
Public Service Company of New Mexico | 2015 Electric Rate Case | Non-Fuel Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Requested rate increase (decrease) |  |  |  |  |  |  |  |  |  |  | $ 121.7 |  |  |  |  |  |  |  |  |
Four Corners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 90.1 | $ 148.1 |
Scenario, forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Pre-tax regulatory disallowance for capital costs |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 36.8 |  | $ 58.0 |  |  |

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