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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash Balances to Cash and Restricted Cash (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | $ 88,204 | $ 106,599 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 218,129 | 264,950 | $ 829,003 | $ 117,063
Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 55,500 | 73,600 |  |
Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 29,700 | 41,200 |  |
Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 44,700 | 43,600 |  |
CL&P |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 62,791 | 90,801 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 71,762 | 99,809 | 17,235 | 4,971
CL&P | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 8,700 | 8,700 |  |
CL&P | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 300 | 300 |  |
CL&P | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
NSTAR Electric |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 3,538 | 102 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 20,838 | 17,410 | 12,409 | 6,312
NSTAR Electric | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 17,200 | 17,200 |  |
NSTAR Electric | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 100 | 100 |  |
NSTAR Electric | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 0 | 0 |  |
PSNH |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Cash as reported on the Balance Sheets | 3,869 | 141 |  |
Cash and Restricted Cash as reported on the Statements of Cash Flows | 26,173 | 39,555 | $ 22,595 | $ 36,688
PSNH | Special Deposits |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 18,600 | 36,800 |  |
PSNH | Marketable Securities |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | 500 | 600 |  |
PSNH | Other Long-Term Assets |  |  |  |
Restricted Cash and Cash Equivalents Items [Line Items] |  |  |  |
Restricted cash | $ 3,200 | $ 2,100 |  |

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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631418-115840 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 852 -SubTopic 10 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 21 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631419-115840 + Details
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