RATE REDUCTION BONDS AND VARIABLE INTEREST ENTITIES - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	$ 5,145,900	$ 5,239,300
Regulatory Liabilities	3,609,475	3,637,273
Accrued Interest (included in Other Current Liabilities)	823,006	738,222
Rate Reduction Bonds - Current Portion	52,332	0
Rate Reduction Bonds - Long-Term Portion	583,331	0
Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	608,400	0
Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	208,000	137,400
Public Service Company Of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	929,500	940,800
Regulatory Liabilities	428,069	444,397
Accrued Interest (included in Other Current Liabilities)	64,046	67,924
Rate Reduction Bonds - Current Portion	52,332	0
Rate Reduction Bonds - Long-Term Portion	583,331	0
Public Service Company Of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)		0
Public Service Company Of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	12,100	15,800
Variable Interest Entity, Primary Beneficiary | Public Service Company Of New Hampshire | Securitized Stranded Costs
Condensed Balance Sheet Statements, Captions [Line Items]
Securitized Stranded Cost (included in Regulatory Assets)	608,400
Variable Interest Entity, Primary Beneficiary | Public Service Company Of New Hampshire | Other Regulatory Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Regulatory Liabilities	5,800
Accrued Interest (included in Other Current Liabilities)	14,400
Prepaid Expenses and Other Current Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	72,100	24,400
Prepaid Expenses and Other Current Assets | Variable Interest Entity, Primary Beneficiary | Public Service Company Of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	47,500	500
Other Noncurrent Assets
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	3,200	0
Other Noncurrent Assets | Variable Interest Entity, Primary Beneficiary | Public Service Company Of New Hampshire
Condensed Balance Sheet Statements, Captions [Line Items]
Restricted Cash and Cash Equivalents	$ 3,200	$ 0