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Financing - Financing Activities (Details) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 09, 2017 | Feb. 01, 2016 | Mar. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Oct. 21, 2016 | Sep. 30, 2015 | Mar. 09, 2015
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  |  | 1.927% |
Estimated principal payments |  |  | $ 283,500,000 | $ 283,500,000 |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Interest rate at period end |  |  | 1.75% | 1.75% |  |  |  |
PNMR 2016 Two-Year Term Loan |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Term loans |  |  | $ 100,000,000.0 | $ 100,000,000.0 |  |  |  |
Interest rate |  |  | 1.93% | 1.93% |  |  |  |
Variable rate short-term debt |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed interest rate | 1.926% |  |  |  |  |  |  |
Aggregate principal amount | $ 50,000,000.0 |  |  |  |  |  |  |
Term of hedging agreement | 4 years |  |  |  |  |  |  |
Cash flow hedge | Level 2 |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fair value gain (loss) |  |  | $ (400,000) | $ 200,000 | $ (100,000) |  |  |
PNM |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Estimated principal payments |  |  | 232,000,000 | $ 232,000,000 |  |  |  |
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] |  |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party |  | $ 125,000,000.0 |  |  |  |  |  |
Line of credit |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  |  | 65.00% |  |  |  |
PNMR 2015 Term Loan Agreement |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  |  | $ 150,000,000.0
PNM 2016 Term Loan Agreement [Member] | PNM |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Aggregate principal amount |  |  | $ 175,000,000.0 | $ 175,000,000.0 |  |  |  |
Interest rate |  |  | 1.59% | 1.59% |  |  |  |
BTMU Term Loan Agreement | NM Capital |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Term loans |  | $ 125,000,000.0 | $ 82,800,000 | $ 82,800,000 |  |  |  |
Stated interest rate |  |  | 3.79% | 3.79% |  |  |  |
Estimated principal payments |  |  | $ 33,500,000 | $ 33,500,000 |  |  |  |
Letter of credit | PNMR | JPM LOC Facility |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Aggregate principal amount |  |  |  |  |  | $ 30,300,000.0 |  |

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