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Financing - Financing Activities (Details) - USD ($) |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 09, 2018 | Jul. 28, 2017 | Feb. 01, 2016 | Jul. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Jul. 25, 2018 | Jun. 28, 2018 | May 22, 2018 | May 14, 2018 | Mar. 09, 2018 | Dec. 31, 2017 | Jul. 20, 2017 | Oct. 21, 2016 | Mar. 09, 2015
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Charges |  |  |  |  | $ 30,492,000 | $ 32,106,000 | $ 96,868,000 | $ 96,137,000 |  |  |  |  |  |  |  |  |
Other deferred credits |  |  |  |  | 127,612,000 |  | $ 127,612,000 |  |  |  |  |  |  | $ 131,706,000 |  |  |
PNMR 2016 One Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of loan |  |  |  | 1 year |  |  | 1 year |  |  |  |  |  |  |  |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term of loan |  |  |  | 2 years |  |  | 2 years |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  | $ 100,000,000.0 |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |
Variable interest rate |  |  |  |  | 3.03% |  | 3.03% |  |  |  |  |  |  |  |  |  |
PNM |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Charges |  |  |  |  | $ 18,063,000 | 20,451,000 | $ 58,881,000 | 62,393,000 |  |  |  |  |  |  |  |  |
Other deferred credits |  |  |  |  | 171,345,000 |  | $ 171,345,000 |  |  |  |  |  |  | 106,442,000 |  |  |
PNM | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maturity term over which financings require regulator approval (more than) |  |  |  |  |  |  | 18 months |  |  |  |  |  |  |  |  |  |
NM Capital | Coal supply | San Juan Coal Company, Westmoreland |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of line of credit |  |  |  |  |  |  |  |  |  |  | $ 50,100,000 |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  |  | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Texas-New Mexico Power Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Charges |  |  |  |  | 8,241,000 | $ 7,704,000 | $ 23,771,000 | $ 22,619,000 |  |  |  |  |  |  |  |  |
Other deferred credits |  |  |  |  | 6,692,000 |  | 6,692,000 |  |  |  |  |  |  | 7,448,000 |  |  |
Repayments from transmission interconnection arrangement |  |  |  |  | 4,100,000 |  | $ 4,100,000 |  |  |  |  |  |  | $ 0 |  |  |
Line of credit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  |  | 70.00% |  |  | 65.00% |  |  |  |  |  |  |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayment of line of credit |  |  | 125,000,000.0 |  |  |  |  |  |  |  | $ 43,000,000 |  |  |  |  |  |
Term loans |  |  | $ 125,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Letter of credit | PNMR | JPM LOC Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,300,000.0 |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 300,000,000.0 |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  | 3.25% |  |  |  |
Senior Unsecured Notes | PNM | Senior Unsecured Note Agreement (SUNs) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  | $ 450,000,000.0 |  |  | 450,000,000 |  | $ 450,000,000 |  |  |  |  |  |  |  |  |  |
Debt-to-Capital ratio |  | 65.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  | $ 350,000,000.0 |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018 | Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | 350,000,000 |  | 350,000,000 |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018 | Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | 100,000,000 |  | 100,000,000 |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.15% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 55,000,000.0 |  | $ 55,000,000.0 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.15% |  | 3.15% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.45% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 104,000,000 |  | $ 104,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.45% |  | 3.45% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.68% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 88,000,000 |  | $ 88,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.68% |  | 3.68% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 3.93% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 38,000,000 |  | $ 38,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.93% |  | 3.93% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.22% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 45,000,000 |  | $ 45,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 4.22% |  | 4.22% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in May 2018, Interest rate of 4.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 20,000,000 |  | $ 20,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 4.50% |  | 4.50% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 3.78% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 15,000,000 |  | $ 15,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.78% |  | 3.78% |  |  |  |  |  |  |  |  |  |
Senior Unsecured Notes | PNM | SUNs, Issuance in August 2018, Interest rate of 4.60% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 85,000,000 |  | $ 85,000,000 |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 4.60% |  | 4.60% |  |  |  |  |  |  |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 150,000,000.0
Mortgages | Texas-New Mexico Power Company | First Mortgage Bonds |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  | $ 172,300,000.0 |  | $ 172,300,000.0 |  |  | $ 60,000,000.0 |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  | 2.94% | 3.85% |  |  |  |  |  |  |
TNMP Term Loan Agreement | Texas-New Mexico Power Company | TNMP Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  | $ 20,000,000.0 |  |  |  |  |  |  |  |
PNM 2017 Term Loan Agreement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate at period end |  |  |  |  | 2.98% |  | 2.98% |  |  |  |  |  |  |  |  |  |
Term loan agreement with banks | PNM | PNM 2017 Term Loan Agreement | JPMorgan Chase Bank, N.A. and U.S. Bank National Association |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 200,000,000.0 |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 20,000,000.0 |  |
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in May 2018, 7.95% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  | 7.95% |  |  |  |  |
Unsecured debt | PNM | Senior Unsecured Notes, Maturity in August 2018, 7.50% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 7.50% |  | 7.50% |  |  |  |  |  |  |  |  |  |
Deposit Related To Potential Transmission Interconnections | PNMR Development |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of related party transaction | $ 68,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest Charges |  |  |  |  | $ 800,000 |  | $ 1,500,000 |  |  |  |  |  |  |  |  |  |
Other deferred credits | $ 68,200,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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