SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	$ 568.7	$ 518.0	$ 532.4
Income Taxes	121.6	48.9	56.0
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	467.9	367.2	379.4
The Connecticut Light and Power Company
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	161.5	149.0	144.6
Income Taxes	38.4	10.9	80.6
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	110.6	101.8	111.3
NSTAR Electric Company
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	141.6	129.4	121.9
Income Taxes	74.2	110.7	77.9
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	120.0	103.2	116.4
Public Service Company of New Hampshire
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	56.5	54.5	56.9
Income Taxes	51.1	34.2	3.4
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	$ 68.7	$ 33.3	$ 49.9