INVESTMENTS IN UNCONSOLIDATED AFFILIATES - Summary of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affilitates	$ 871,633	$ 464,286
Offshore Wind Business - North East Offshore and Bay State Wind
Schedule of Equity Method Investments [Line Items]
Ownership interest	50.00%
Investments in unconsolidated affilitates	$ 649,300	234,300
Natural Gas Pipeline - Algonquin Gas Transmission, LLC
Schedule of Equity Method Investments [Line Items]
Ownership interest	15.00%
Investments in unconsolidated affilitates	$ 127,800	155,000
Renewable Energy Investment Fund
Schedule of Equity Method Investments [Line Items]
Ownership interest	90.00%
Investments in unconsolidated affilitates	$ 72,400	54,100
Other
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated affilitates	$ 22,100	$ 20,900