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Financing - Financing Activities (Details) - USD ($) |  |  |  | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 01, 2019 | Jan. 18, 2019 | Apr. 09, 2018 | Mar. 31, 2019 | Mar. 31, 2018 | Jun. 30, 2018 | Jul. 01, 2019 | Mar. 29, 2019 | Feb. 26, 2019 | Dec. 31, 2018 | Jul. 25, 2018 | Jul. 20, 2017 | Oct. 21, 2016
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest charges |  |  |  | $ 31,634,000 | $ 33,055,000 |  |  |  |  |  |  |  |
Other deferred credits |  |  |  | $ 169,372,000 |  |  |  |  |  | $ 167,668,000 |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  | 3.29% |  |  |  |  |  |  |  |  |
Term loans |  |  |  | $ 50,000,000 |  |  |  |  |  |  |  |  |
Term of loan |  |  |  | 2 years |  |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest charges |  |  |  | $ 18,360,000 | 20,830,000 |  |  |  |  |  |  |  |
Other deferred credits |  |  |  | $ 215,004,000 |  |  |  |  |  | 215,737,000 |  |  |
PNM | PNM 2019 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 250,000,000.0 |  |  |  |  |  |  |  |  |  |  |
PNM | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity term over which financings require regulator approval (more than) |  |  |  | 18 months |  |  |  |  |  |  |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest charges |  |  |  | $ 8,800,000 | $ 7,729,000 |  |  |  |  |  |  |  |
Other deferred credits |  |  |  | $ 3,729,000 |  |  |  |  |  | $ 2,908,000 |  |  |
Texas-New Mexico Power Company | TNMP 2019 Bond Purchase Agreement |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 225,000,000 | $ 305,000,000.0 |  |  |  |
Texas-New Mexico Power Company | First Mortgage Bonds Due 2019, Series 2009A, at 9 point 50 percent |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt |  | $ 172,300,000 |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  | 9.50% |  |  |  |  |  |  |  |  |  |  |
Line of credit |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  |  | 65.00% |  | 70.00% |  |  |  |  |  |  |
Letter of credit | PNMR | JPM LOC Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,300,000.0
PNM 2019 Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate at period end |  |  |  | 3.13% |  |  |  |  |  |  |  |  |
Unsecured Debt | Texas-New Mexico Power Company | First Mortgage Bonds Due 2019, Series 2009A, at 9 point 50 percent |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Extinguishment of debt | $ 172,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate | 9.50% |  |  |  |  |  |  |  |  |  |  |  |
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds 3.79 Percent Due 2034 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 75,000,000 |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 3.79% |  |  |  |  |
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds 3.92 Percent Due 2039 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 75,000,000 |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 3.92% |  |  |  |  |
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds 4.06 Percent Due 2044 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  | $ 75,000,000 |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 4.06% |  |  |  |  |
Mortgages | Texas-New Mexico Power Company | TNMP 2019 Bond Purchase Agreement |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 305,000,000 |  |  |  | $ 225,000,000 | $ 305,000,000.0 |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  | 400.00% |  |  |  |
Debt-to-capital ratio |  | 65.00% |  |  |  |  |  |  |  |  |  |  |
Mortgages | Texas-New Mexico Power Company | TNMP Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  | 3.20% |  |
Mortgages | Texas-New Mexico Power Company | PNMR Development Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  | 3.30% |  |  |  |  |  |  |  |  |
Term loan agreement with banks | PNM | PNM 2019 Term Loan | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 250,000,000 |
Term loan agreement with banks | Texas-New Mexico Power Company | TNMP Term Loan Agreement | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 35,000,000 |
Term loan agreement with banks | Texas-New Mexico Power Company | PNMR Development Term Loan | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 90,000,000.0 |  |  |  |  |  |  |  |  |
Subsequent event | Texas-New Mexico Power Company | First Mortgage Bonds 3.60 Percent Due 2029 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 80,000,000 |  |  |  |  |  |
Subsequent event | Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds 3.60 Percent Due 2029 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  | $ 80,000,000 |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  | 3.60% |  |  |  |  |  |
Deposit Related To Potential Transmission Interconnections | PNMR Development |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of related party transaction |  |  | $ 68,200,000 |  |  |  |  |  |  |  |  |  |
Interest charges |  |  |  | 900,000 |  |  |  |  |  |  |  |  |
Other deferred credits |  |  |  | $ 68,200,000 |  |  |  |  |  |  |  |  |

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