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Financing - Schedule of Maturities and Interest Rates (Details) - Texas-New Mexico Power Company - Mortgages - Subsequent Event - USD ($) | Jul. 01, 2019 | Mar. 29, 2019 | Feb. 26, 2019
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First Mortgage Bonds 3.79 Percent Due 2034 |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 75,000,000 |
Stated percentage |  | 3.79% |
First Mortgage Bonds 3.92 Percent Due 2039 |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 75,000,000 |
Stated percentage |  | 3.92% |
First Mortgage Bonds 4.06 Percent Due 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 75,000,000 |
Stated percentage |  | 4.06% |
TNMP 2019 Bond Purchase Agreement |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount |  | $ 225,000,000 | $ 305,000,000.0
Stated percentage |  |  | 400.00%
First Mortgage Bonds 3.60 Percent Due 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Aggregate principal amount | $ 80,000,000 |  |
Stated percentage | 3.60% |  |

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