INVESTMENTS IN UNCONSOLIDATED AFFILIATES - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) company	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 08, 2019 mi²
Schedule of Equity Method Investments [Line Items]
Equity in earnings of unconsolidated affiliates, net of impairment	$ 14,200	$ 42,200	$ 3,800
Dividends received from equity method investees	$ 21,800	48,900	22,300
Number of companies that transmit hydro electricity imported from The Hydro-Quebec System in Canada | company	2
Payments to acquire investments	$ 239,673	416,337	205,150
Offshore Wind Business - North East Offshore and Bay State Wind
Schedule of Equity Method Investments [Line Items]
Carrying amount of equity method investments in excess of the underlying equity in net assets	264,100	240,300
Goodwill	$ 168,300
Ownership interest	50.00%
Revolution Wind And South Fork Wind
Schedule of Equity Method Investments [Line Items]
Ownership interest				50.00%
Area of land (in square miles) | mi²				257
Bay State Wind
Schedule of Equity Method Investments [Line Items]
Ownership interest				50.00%
Area of land (in square miles) | mi²				300
Hydro-Quebec System
Schedule of Equity Method Investments [Line Items]
Ownership interest	14.50%
Payments to acquire investments	$ 8,600	$ 8,200
Renewable Energy Fund
Schedule of Equity Method Investments [Line Items]
Other-than-temporary impairment			2,800
Access Northeast Project
Schedule of Equity Method Investments [Line Items]
Other-than-temporary impairment			$ 32,900