SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	$ 532.4	$ 503.2	$ 419.1
Income Taxes	56.0	158.8	30.8
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	379.4	389.3	379.5
CL&P
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	144.6	149.7	144.6
Income Taxes	80.6	66.1	68.8
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	111.3	106.1	132.5
NSTAR Electric
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	121.9	122.1	124.6
Income Taxes	77.9	120.0	95.5
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	116.4	116.5	116.5
PSNH
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	56.9	40.5	45.9
Income Taxes	3.4	27.3	26.1
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	$ 49.9	$ 35.1	$ 44.4