SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	$ 518.0	$ 532.4	$ 503.2
Income Taxes	48.9	56.0	158.8
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	367.2	379.4	389.3
The Connecticut Light and Power Company
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	149.0	144.6	149.7
Income Taxes	10.9	80.6	66.1
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	101.8	111.3	106.1
NSTAR Electric Company
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	129.4	121.9	122.1
Income Taxes	110.7	77.9	120.0
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	103.2	116.4	116.5
Public Service Company of New Hampshire
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	54.5	56.9	40.5
Income Taxes	34.2	3.4	27.3
Non-Cash Investing Activities:
Plant additionsincluded in accounts payable (as of)	$ 33.3	$ 49.9	$ 35.1