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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) customer in Millions | 9 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2022 USD ($) regulatedUtility customer project | Jun. 30, 2022 USD ($) | May 04, 2022 | Mar. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Sep. 30, 2021 USD ($) | Jun. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates | $ 2,053,404,000 |  |  |  | $ 1,436,293,000 |  |  |
Number of electric, natural gas and water customers | customer | 4.4 |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | $ 457,600,000 | $ 457,800,000 |  |  | 417,400,000 | $ 441,700,000 | $ 425,800,000 | $ 358,900,000
Number of offshore wind projects | project | 3 |  |  |  |  |  |  |
Cash, cash equivalents, restricted cash and restricted cash equivalents | $ 602,256,000 |  |  |  | $ 221,008,000 | 218,129,000 |  | 264,950,000
Restricted Cash, Noncurrent, Statement of Financial Position [Extensible Enumeration] | Other Long-Term Assets |  |  |  | Other Long-Term Assets |  |  |
Energy Relief Fund - Merrimack Valley Programs |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Restricted cash, current | $ 20,000,000 |  |  |  |  |  |  |
Restricted cash, noncurrent | 15,900,000 |  |  |  |  |  |  |
Restricted cash |  |  |  |  | $ 41,500,000 |  |  |
COVID-19 |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 55,500,000 |  |  |  | 55,300,000 |  |  |
CL&P |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | $ 198,600,000 | 192,700,000 |  |  | 181,300,000 | 195,500,000 | 188,800,000 | 157,400,000
Period of accounts receivable recoverable under financial or medical duress | 180 days |  |  |  |  |  |  |
Cash, cash equivalents, restricted cash and restricted cash equivalents | $ 23,120,000 |  |  |  | 74,788,000 | 71,762,000 |  | 99,809,000
CL&P | Customer Assistance Fund |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Cash, cash equivalents, restricted cash and restricted cash equivalents |  |  |  | $ 10,000,000 | 10,000,000 |  |  |
CL&P | COVID-19 |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 19,700,000 |  |  |  | 23,900,000 |  |  |
NSTAR Electric |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 99,500,000 | 100,500,000 |  |  | 97,000,000.0 | 105,000,000.0 | 98,000,000.0 | 91,600,000
Cash, cash equivalents, restricted cash and restricted cash equivalents | 470,945,000 |  |  |  | 18,179,000 | 20,838,000 |  | 17,410,000
NSTAR Electric | COVID-19 |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 4,100,000 |  |  |  | 9,000,000 |  |  |
PSNH |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | 28,800,000 | $ 27,300,000 |  |  | 24,300,000 | 17,000,000.0 | $ 17,200,000 | 17,200,000
Cash, cash equivalents, restricted cash and restricted cash equivalents | 24,246,000 |  |  |  | 35,126,000 | $ 26,173,000 |  | $ 39,555,000
PSNH | COVID-19 |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Allowance for uncollectible accounts for late fees and other receivable amounts | $ 0 |  |  |  | $ 0 |  |  |
Yankee Gas |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Period of accounts receivable recoverable under financial or medical duress | 90 days |  |  |  |  |  |  |
Connecticut, Massachusetts and New Hampshire |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Number of regulated utilities | regulatedUtility | 12 |  |  |  |  |  |  |
Offshore Wind Business |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Ownership interest |  |  | 50.00% |  |  |  |  |
North East Offshore LLC |  |  |  |  |  |  |  |
Summary of Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Ownership interest | 50.00% |  |  |  |  |  |  |

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