SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 224,800
Uncollectible Expense	11,353	$ 18,565
Uncollectible Costs Deferred	20,300
Write-Offs	(21,500)
Recoveries Collected	3,300
Ending Balance	262,100
CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	97,300
Uncollectible Expense	3,230	4,116
Uncollectible Costs Deferred	14,700
Write-Offs	(7,300)
Recoveries Collected	800
Ending Balance	130,900
NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	75,400
Uncollectible Expense	3,704	5,953
Uncollectible Costs Deferred	0
Write-Offs	(8,400)
Recoveries Collected	1,700
Ending Balance	71,100
PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	10,500
Uncollectible Expense	666	$ 1,725
Uncollectible Costs Deferred	1,300
Write-Offs	(1,700)
Recoveries Collected	100
Ending Balance	11,200
Revision of Prior Period, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	23,800
Revision of Prior Period, Adjustment | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	22,200
Revision of Prior Period, Adjustment | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	(1,300)
Revision of Prior Period, Adjustment | PSNH
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	300
Hardship Accounts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	143,300
Uncollectible Expense	0
Uncollectible Costs Deferred	11,800
Write-Offs	(4,700)
Recoveries Collected	200
Ending Balance	172,200
Hardship Accounts | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	80,100
Uncollectible Expense	0
Uncollectible Costs Deferred	12,900
Write-Offs	(3,800)
Recoveries Collected	200
Ending Balance	110,700
Hardship Accounts | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	43,900
Uncollectible Expense	0
Uncollectible Costs Deferred	(3,400)
Write-Offs	(400)
Recoveries Collected	0
Ending Balance	38,500
Hardship Accounts | Revision of Prior Period, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,600
Hardship Accounts | Revision of Prior Period, Adjustment | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	21,300
Hardship Accounts | Revision of Prior Period, Adjustment | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	(1,600)
Retail (Non-Hardship), Wholesale, and Other Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	81,500
Uncollectible Expense	11,400
Uncollectible Costs Deferred	8,500
Write-Offs	(16,800)
Recoveries Collected	3,100
Ending Balance	89,900
Retail (Non-Hardship), Wholesale, and Other Receivables | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	17,200
Uncollectible Expense	3,200
Uncollectible Costs Deferred	1,800
Write-Offs	(3,500)
Recoveries Collected	600
Ending Balance	20,200
Retail (Non-Hardship), Wholesale, and Other Receivables | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	31,500
Uncollectible Expense	3,700
Uncollectible Costs Deferred	3,400
Write-Offs	(8,000)
Recoveries Collected	1,700
Ending Balance	32,600
Retail (Non-Hardship), Wholesale, and Other Receivables | Revision of Prior Period, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,200
Retail (Non-Hardship), Wholesale, and Other Receivables | Revision of Prior Period, Adjustment | CL&P
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	900
Retail (Non-Hardship), Wholesale, and Other Receivables | Revision of Prior Period, Adjustment | NSTAR Electric
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 300