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Financing - Financing Activities (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 01, 2016 | Dec. 31, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Dec. 17, 2015 | Sep. 30, 2015 | Mar. 09, 2015
Debt Instrument [Line Items] |  |  |  |  |  |  |
Fixed interest rate |  |  |  |  |  | 1.927% |
Cash Flow Hedging [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Fair value gain (loss) |  |  | $ (1,100,000) | $ 100,000 |  |  |
NM Capital [Member] | Coal Supply [Member] | San Juan Generating Station [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Payments to fund long-term loans to unaffiliated third party | $ 125,000,000.0 |  |  |  |  |  |
Maximum [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Maturity term over which financings require regulator approval |  |  | 18 months |  |  |  |
Line of Credit [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Ratio of debt to capital (less than or equal to) |  |  | 65.00% |  |  |  |
PNMR 2015 Term Loan Agreement [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Term loans |  |  |  |  |  |  | $ 150,000,000.0
Interest rate |  |  | 1.35% |  |  |  |
BTMU Term Loan Agreement [Member] | NM Capital [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Term loans | $ 125,000,000.0 |  |  |  |  |  |
Stated interest rate |  |  | 3.37% |  |  |  |
First Mortgage Bonds [Member] | Texas-New Mexico Power Company [Member] | First Mortgage Bonds, 3.53%, due 2026, Series 2016A [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Stated interest rate |  |  |  |  | 3.53% |  |
Aggregate principal amount |  |  |  |  | $ 60,000,000.0 |  |
PNM 2014 Multi-Draw Term Loan Agreement [Member] | Public Service Company of New Mexico [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Term loans |  |  | $ 125,000,000 |  |  |  |
Interest rate |  |  | 1.02% |  |  |  |
Subsequent Event [Member] | BTMU Term Loan Agreement [Member] | Minimum [Member] | NM Capital [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt, required quarterly payment |  | $ 5,000,000.0 |  |  |  |  |

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