SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	$ 108,068	$ 38,165
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	209,324	85,890	$ 106,750	$ 67,996
The Connecticut Light And Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	87,721	6,028
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	91,613	9,619	8,403	2,205
NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	1,606	1,763
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	14,659	14,708	15,507	12,821
Public Service Company Of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and Cash Equivalents	1,439	900
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	52,723	2,191	5,953	$ 4,088
Prepaid Expenses and Other Current Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	72,100	24,400
Prepaid Expenses and Other Current Assets | The Connecticut Light And Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	3,500	3,100
Prepaid Expenses and Other Current Assets | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	13,000	12,800
Marketable Securities
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	25,900	23,300
Marketable Securities | The Connecticut Light And Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	400	500
Marketable Securities | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	100	100
Marketable Securities | Public Service Company Of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	600	800
Other Noncurrent Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	3,200	0
Other Noncurrent Assets | The Connecticut Light And Power Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	0	0
Other Noncurrent Assets | NSTAR Electric Company
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	0	0
Variable Interest Entity, Primary Beneficiary | Prepaid Expenses and Other Current Assets | Public Service Company Of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	47,500	500
Variable Interest Entity, Primary Beneficiary | Other Noncurrent Assets | Public Service Company Of New Hampshire
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents	$ 3,200	0
Accounting Standards Update 2016-18
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted Cash and Cash Equivalents		$ 28,800	$ 32,400