SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	$ 783.2	$ 636.2	$ 568.7
Income Taxes	39.2	77.9	121.6
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	564.1	586.9	467.9
CL&P
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	176.8	167.2	161.5
Income Taxes	(44.1)	117.6	38.4
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	139.8	131.8	110.6
NSTAR Electric
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	182.8	152.8	141.6
Income Taxes	31.3	23.8	74.2
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	178.9	184.3	120.0
PSNH
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	62.8	58.3	56.5
Income Taxes	(59.9)	58.3	51.1
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	$ 65.9	$ 76.2	$ 68.7