SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Non-cash Investing Activities (Details) - USD ($) $ in Millions |
6 Months Ended | |
|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
|
| Schedule of Supplemental Cash Flow [Line Items] | ||
| Plant additions included in accounts payable | $ 405.1 | $ 477.9 |
| CL&P | ||
| Schedule of Supplemental Cash Flow [Line Items] | ||
| Plant additions included in accounts payable | 73.1 | 89.2 |
| NSTAR Electric | ||
| Schedule of Supplemental Cash Flow [Line Items] | ||
| Plant additions included in accounts payable | 124.6 | 142.2 |
| PSNH | ||
| Schedule of Supplemental Cash Flow [Line Items] | ||
| Plant additions included in accounts payable | $ 67.7 | $ 86.5 |