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REGULATORY ACCOUNTING - Narrative (Details) - USD ($) | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Oct. 01, 2021 | Sep. 01, 2021 | May 06, 2021 | May 08, 2018
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | $ 252,500,000 | $ 196,900,000 |  |  |  |
Regulatory assets | 5,715,800,000 | 6,569,900,000 |  |  |  |
Deferred costs related to uncollectible expense | 33,000,000 | 15,800,000 |  |  |  |
Regulatory Liabilities | 602,432,000 | 389,430,000 |  |  |  |
FERC ROE Complaints |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers | 39,100,000 |  |  |  |  |
COVID 19 |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Deferred costs | 39,800,000 | 24,000,000 |  |  |  |
Deferred Storm Restoration Costs |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 1,010,000,000.00 |  |  |  |  |
Deferred Storm Restoration Costs | Tropical Storm Isaias |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | $ 251,000,000 |  |  |  |  |
Goodwill-related |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Recovery period | 40 years |  |  |  |  |
Remaining amortization | 18 years |  |  |  |  |
Regulatory assets | $ 297,800,000 | 314,700,000 |  |  |  |
Storm Costs, Net |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 1,102,700,000 | 765,600,000 |  |  |  |
Public Service Company of New Hampshire |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 3,400,000 | 4,300,000 |  |  |  |
Regulatory assets | 786,400,000 | 989,100,000 |  |  |  |
Equity return not recorded on the balance sheet | 5,000,000 | 5,100,000 |  |  |  |
Regulatory Liabilities | 120,176,000 | 58,756,000 |  |  |  |
Public Service Company of New Hampshire | FERC ROE Complaints |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers | 3,100,000 |  |  |  |  |
Public Service Company of New Hampshire | Deferred Storm Restoration Costs |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 61,000,000 |  |  |  |  |
Public Service Company of New Hampshire | Goodwill-related |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 0 | 0 |  |  |  |
Public Service Company of New Hampshire | Storm Costs, Net |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 65,800,000 | 64,100,000 |  |  |  |
The Connecticut Light and Power Company |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 114,900,000 | 84,100,000 |  |  |  |
Regulatory assets | 2,084,800,000 | 2,211,800,000 |  |  |  |
Deferred costs related to uncollectible expense | 19,000,000 | 4,700,000 |  |  |  |
Equity return not recorded on the balance sheet |  | 200,000 |  |  |  |
Regulatory Liabilities | 266,489,000 | 137,166,000 |  |  |  |
The Connecticut Light and Power Company | CL&P Rate Adjustment Mechanisms (RAM) |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Recovery of cumulative under-recoveries |  | 193,000,000 |  |  |  |
The Connecticut Light and Power Company | FERC ROE Complaints |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers | 21,400,000 |  |  |  |  |
The Connecticut Light and Power Company | Unfavorable Regulatory Action |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory Liabilities | 75,000,000 |  |  |  |  |
Loss contingency, estimate of possible earnings impact, non-compliance with performance standards |  |  |  | $ 28,400,000 | $ 28,400,000 |
Customer credits |  |  | $ 65,000,000 |  |  |
Customer assistance initiatives |  |  | $ 10,000,000 |  |  |
The Connecticut Light and Power Company | Deferred Pre-Staging And Storm Restoration Costs |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 232,000,000 | 344,000,000 |  |  |  |
The Connecticut Light and Power Company | Deferred Storm Restoration Costs |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 643,000,000 |  |  |  |  |
The Connecticut Light and Power Company | Deferred Storm Restoration Costs | Tropical Storm Isaias |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 234,000,000 |  |  |  |  |
The Connecticut Light and Power Company | Goodwill-related |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 0 | 0 |  |  |  |
The Connecticut Light and Power Company | Storm Costs, Net |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 695,600,000 | 515,100,000 |  |  |  |
NSTAR Electric Company |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 85,000,000 | 69,800,000 |  |  |  |
Regulatory assets | 1,579,200,000 | 1,703,900,000 |  |  |  |
Deferred costs related to uncollectible expense | 11,200,000 | 11,900,000 |  |  |  |
Regulatory Liabilities | 228,248,000 | 164,761,000 |  |  |  |
NSTAR Electric Company | FERC ROE Complaints |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Cumulative pre-tax reserves which include impact of refunds given to customers | 14,600,000 |  |  |  |  |
NSTAR Electric Company | Deferred Storm Restoration Costs |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of regulatory costs not yet approved | 308,000,000 |  |  |  |  |
Amount of regulatory recovery not yet approved |  | 7,200,000 |  |  |  |
Pre-tax benefit to earnings from settlement agreement | 15,600,000 | 15,600,000 |  |  |  |
NSTAR Electric Company | Goodwill-related |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | 255,700,000 | 270,200,000 |  |  |  |
NSTAR Electric Company | Storm Costs, Net |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Regulatory assets | $ 341,300,000 | $ 186,400,000 |  |  |  |
Rate Reduction Bonds | Public Service Company of New Hampshire |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |
Amount of securitized rate reduction bonds issued |  |  |  |  |  | $ 635,700,000

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