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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions |  | 12 Months Ended
--- | --- | ---
Oct. 09, 2020 USD ($) | Dec. 31, 2021 USD ($) customer utility MW | Dec. 31, 2020 USD ($) MW | Dec. 31, 2019 USD ($) $ / shares
Summary of Accounting Policies [Line Items] |  |  |  |
Number of electric and natural gas customers | customer |  | 4.4 |  |
Goodwill |  | $ 4,477,269 | $ 4,445,988 | $ 4,427,000
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW |  |  | 675 |
Pre-tax impairment charge |  | 0 | $ 0 | 239,644
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 417,406 | 358,851 | 224,800
Charitable contributions |  |  | 6,400 |
Increase to operating cash inflows in accounts payable |  | 29,201 | (122,567) | (14,866)
Increase to investing cash outflows in investments in property, plant and equipment |  | 3,175,080 | 2,942,996 | 2,911,489
Electric Transmission |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Goodwill |  | 577,000 | 577,000 | 577,000
Energy Relief Fund |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Restricted cash |  | $ 41,500 |  |
Northern Pass Transmission |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Amount of power to be purchased under capacity-related contract (up to) (in MW) | MW |  | 1,090 |  |
Pre-tax impairment charge |  |  |  | 318,000
After-tax impact of impairment charge |  |  |  | $ (204,400)
After-tax impact of impairment charge (in dollars per share) | $ / shares |  |  |  | $ (0.64)
Northern Pass Transmission | Electric Transmission |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Pre-tax impairment charge |  |  |  | $ 239,600
Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | $ 191,300 | 164,100 | 81,500
CMA |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Purchase price | $ 1,100,000 |  |  |
Target working capital amount | 68,600 |  |  |
Goodwill | $ 52,000 |  |  |
The Connecticut Light and Power Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | $ 181,319 | 157,447 | 97,300
Period of accounts receivable recoverable under financial or medical duress |  | 180 days |  |
Maximum loans receivable outstanding |  | $ 55,000 |  |
Loans transferred receivable current |  | 10,500 | 12,900 |
Loans transferred receivable non-current |  | 8,300 | 9,500 |
Tax remittance to state for energy efficiency funds |  |  |  | 21,400
Long term receivable |  | 25,000 | 25,000 |
Increase to operating cash inflows in accounts payable |  | (24,895) | 17,028 | (102,344)
Increase to investing cash outflows in investments in property, plant and equipment |  | 790,083 | 833,973 | 917,532
The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | $ 36,700 | 28,300 | 17,200
Yankee Gas Services Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Period of accounts receivable recoverable under financial or medical duress |  | 90 days |  |
NSTAR Electric Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | $ 97,005 | 91,583 | 75,400
Long term receivable |  | 5,500 | 5,500 |
Increase to operating cash inflows in accounts payable |  | (31,650) | (25,573) | 22,659
Increase to investing cash outflows in investments in property, plant and equipment |  | 960,949 | 907,000 | 861,391
NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 53,700 | 51,900 | 31,500
Public Service Company of New Hampshire |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 24,331 | 17,157 | 10,500
Long term receivable |  | 3,800 | 3,800 |
Increase to operating cash inflows in accounts payable |  | 3,256 | (27,270) | (12,281)
Increase to investing cash outflows in investments in property, plant and equipment |  | $ 326,379 | 342,586 | 308,993
Connecticut, Massachusetts and New Hampshire |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Number of regulated utilities | utility |  | 10 |  |
Restatement Adjustment |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  | 23,800 |
Restatement Adjustment | Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  |  | 2,200
Restatement Adjustment | The Connecticut Light and Power Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  | 22,200 |
Restatement Adjustment | The Connecticut Light and Power Company | Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  |  | 900
Restatement Adjustment | NSTAR Electric Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  | (1,300) |
Restatement Adjustment | NSTAR Electric Company | Retail Non Hardship Wholesaleand Other Receivables |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  |  | $ 300
Restatement Adjustment | Public Service Company of New Hampshire |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  |  | $ 300 |
COVID 19 |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Accounts receivable, allowance for uncollectible accounts increase (decrease) |  | $ 24,100 |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 55,300 |  |
COVID 19 | The Connecticut Light and Power Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Accounts receivable, allowance for uncollectible accounts increase (decrease) |  | 20,100 |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 23,900 |  |
COVID 19 | NSTAR Electric Company |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Accounts receivable, allowance for uncollectible accounts increase (decrease) |  | (1,300) |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | 9,000 |  |
COVID 19 | Public Service Company of New Hampshire |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Decrease in deferral cost |  | 600 |  |
COVID 19 | Natural Gas Businesses |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |
Accounts receivable, allowance for uncollectible accounts increase (decrease) |  | 6,600 |  |
Allowance for uncollectible accounts for late fees and other receivable amounts |  | $ 21,400 |  |

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