SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	$ 398.1	$ 365.9	$ 349.6
Income Taxes	(135.5)	10.3	334.2
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	301.5	216.6	181.9
The Connecticut Light And Power Company [Member]
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	143.3	144.4	144.1
Income Taxes	(73.9)	55.2	135.4
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	116.2	76.0	63.5
NSTAR Electric Company [Member]
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	88.2	75.7	75.3
Income Taxes	80.7	(19.8)	217.1
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	60.9	23.5	34.6
Public Service Company Of New Hampshire [Member]
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	46.5	42.3	41.1
Income Taxes	(36.0)	14.4	2.3
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	37.9	46.5	39.3
Western Massachusetts Electric Company [Member]
Cash Paid During the Year for:
Interest, Net of Amounts Capitalized	24.7	26.7	25.9
Income Taxes	(14.7)	14.7	25.1
Non-Cash Investing Activities:
Plant Additions Included in Accounts Payable (As of)	$ 26.1	$ 27.0	$ 14.2