SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ / shares in Units, $ in Thousands, customer in Millions		3 Months Ended	12 Months Ended
	Feb. 26, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) utility customer $ / shares MW	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Summary of Accounting Policies [Line Items]
Number of electric and natural gas customers | customer			4,000,000
Increase to operating cash inflows in accounts payable			$ (14,866)	$ (24,481)	$ (56,067)
Increase to investing cash outflows in investments in property, plant and equipment			2,911,489	2,569,936	2,348,105
Goodwill			4,427,266	4,427,266
Operating lease liabilities			49,900
Operating lease right-of-use-assets			49,900
Pre-tax impairment charge		$ 239,600	239,644	0	0
After-tax impact of impairment charge			$ 204,400
After-tax impact of impairment charge (in dollars per share) | $ / shares			$ 0.64
Columbia Gas of Massachusetts (CMA) | Subsequent Event
Summary of Accounting Policies [Line Items]
Purchase price	$ 1,100,000
CL&P
Summary of Accounting Policies [Line Items]
Increase to operating cash inflows in accounts payable			$ (102,344)	76,924	(48,032)
Increase to investing cash outflows in investments in property, plant and equipment			917,532	864,136	824,383
Operating lease liabilities			700
Operating lease right-of-use-assets			$ 700
Period of accounts receivable recoverable under financial or medical duress			180 days
Transfer of loans receivable				41,300
Maximum loans receivable outstanding			$ 55,000
Loans transferred receivable current			16,500	18,500
Loans transferred receivable non-current			18,200	22,800
Long term receivable			$ 25,000	25,000
Term of energy efficiency customer loans			3 years
Yankee Gas Services Company
Summary of Accounting Policies [Line Items]
Period of accounts receivable recoverable under financial or medical duress			90 days
NSTAR Electric
Summary of Accounting Policies [Line Items]
Increase to operating cash inflows in accounts payable			$ 22,659	(37,140)	20,421
Increase to investing cash outflows in investments in property, plant and equipment			861,391	725,766	719,623
Operating lease liabilities			24,200
Operating lease right-of-use-assets			24,200
Long term receivable			5,500	5,500
PSNH
Summary of Accounting Policies [Line Items]
Increase to operating cash inflows in accounts payable			(12,281)	10,147	(29,453)
Increase to investing cash outflows in investments in property, plant and equipment			308,993	323,910	$ 312,720
Operating lease liabilities			400
Operating lease right-of-use-assets			400
Long term receivable			$ 3,800	3,800
Connecticut, Massachusetts and New Hampshire
Summary of Accounting Policies [Line Items]
Number of regulated utilities | utility			8
Northern Pass
Summary of Accounting Policies [Line Items]
Capacity required for transmission line | MW			1,090
Capitalized costs			$ 318,000
Accounting Standards Update 2016-02
Summary of Accounting Policies [Line Items]
Operating lease liabilities						$ 58,000
Operating lease right-of-use-assets						58,000
Accounting Standards Update 2016-02 | CL&P
Summary of Accounting Policies [Line Items]
Operating lease liabilities						600
Operating lease right-of-use-assets						600
Accounting Standards Update 2016-02 | NSTAR Electric
Summary of Accounting Policies [Line Items]
Operating lease liabilities						25,300
Operating lease right-of-use-assets						25,300
Accounting Standards Update 2016-02 | PSNH
Summary of Accounting Policies [Line Items]
Operating lease liabilities						600
Operating lease right-of-use-assets						$ 600
Restatement Adjustment
Summary of Accounting Policies [Line Items]
Increase to operating cash inflows in accounts payable				(46,600)
Increase to investing cash outflows in investments in property, plant and equipment				46,600
Restatement Adjustment | PSNH
Summary of Accounting Policies [Line Items]
Increase to operating cash inflows in accounts payable				(46,600)
Increase to investing cash outflows in investments in property, plant and equipment				$ 46,600