Income Taxes (Details) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2026 |
Dec. 31, 2017 |
|
| Income Tax Contingency [Line Items] | ||
| Effective tax rate | 14.69% | |
| Sale of equity method investee, tax effect | $ 0.4 | |
| Tax benefits on merger-related costs | 2.0 | |
| Deferred federal, state and local, income taxes | $ 26.2 | |
| NMPRC | ||
| Income Tax Contingency [Line Items] | ||
| Period of time for proposed return to customers the benefit of the reduction in federal corporate income tax rate | 4 years | 23 years |
| Unprotected excess deferred income taxes | $ 62.7 | |
| PNM | ||
| Income Tax Contingency [Line Items] | ||
| Effective tax rate | 12.28% | |
| Sale of equity method investee, tax effect | $ 0.3 | |
| Tax benefits on merger-related costs | 0.7 | |
| Deferred federal, state and local, income taxes | $ 23.5 | |
| TNMP | ||
| Income Tax Contingency [Line Items] | ||
| Effective tax rate | 20.22% | |
| Sale of equity method investee, tax effect | $ 0.1 | |
| Tax benefits on merger-related costs | 0.0 | |
| Deferred federal, state and local, income taxes | $ 2.7 |