SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized	$ 419.1	$ 398.1	$ 365.9
Income Taxes	30.8	(135.5)	10.3
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	379.5	301.5	216.6
The Connecticut Light And Power Company
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized	144.6	143.3	144.4
Income Taxes	68.8	(73.9)	55.2
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	132.5	116.2	76.0
NSTAR Electric Company
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized	124.6	112.9	102.4
Income Taxes	95.5	66.0	(5.1)
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	116.5	87.0	50.5
Public Service Company Of New Hampshire
Cash Paid/(Received) During the Year for:
Interest, Net of Amounts Capitalized	45.9	46.5	42.3
Income Taxes	26.1	(36.0)	14.4
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	$ 44.4	$ 37.9	$ 46.5