SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Supplemental Cash Flow [Line Items]
Cash and Cash Equivalents	$ 108,068	$ 108,068	$ 38,165
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized		503,200	419,100	$ 398,100
Income Taxes		158,800	30,800	(135,500)
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)		435,900	379,500	301,500
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	209,324	209,324	85,890	106,750	$ 67,996
DOE settlement payment	145,000	(145,000)	0	59,804
The Connecticut Light And Power Company
Schedule of Supplemental Cash Flow [Line Items]
Cash and Cash Equivalents	87,721	87,721	6,028
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized		149,700	144,600	143,300
Income Taxes		66,100	68,800	(73,900)
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)		106,100	132,500	116,200
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	91,613	91,613	9,619	8,403	2,205
NSTAR Electric Company
Schedule of Supplemental Cash Flow [Line Items]
Cash and Cash Equivalents	1,606	1,606	1,763
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized		122,100	124,600	112,900
Income Taxes		120,000	95,500	66,000
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)		116,500	116,500	87,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14,659	14,659	14,708	15,507	12,821
Public Service Company Of New Hampshire
Schedule of Supplemental Cash Flow [Line Items]
Cash and Cash Equivalents	1,439	1,439	900
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized		40,500	45,900	46,500
Income Taxes		27,300	26,100	(36,000)
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)		81,700	44,400	37,900
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	52,723	52,723	2,191	$ 5,953	$ 4,088
Prepaid Expenses and Other Current Assets
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	72,100	72,100	24,400
Prepaid Expenses and Other Current Assets | The Connecticut Light And Power Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	3,500	3,500	3,100
Prepaid Expenses and Other Current Assets | NSTAR Electric Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	13,000	13,000	12,800
Marketable Securities
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	25,900	25,900	23,300
Marketable Securities | The Connecticut Light And Power Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	400	400	500
Marketable Securities | NSTAR Electric Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	100	100	100
Marketable Securities | Public Service Company Of New Hampshire
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	600	600	800
Other Noncurrent Assets
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	3,200	3,200	0
Other Noncurrent Assets | The Connecticut Light And Power Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	0	0	0
Other Noncurrent Assets | NSTAR Electric Company
Non-Cash Investing Activities:
Restricted Cash and Cash Equivalents	$ 0	$ 0	$ 0