LONG-TERM DEBT - Long-Term Debt Issuance Authorization (Details) - USD ($) $ in Thousands	Jan. 04, 2017	Dec. 31, 2016	Dec. 28, 2016	Nov. 18, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term debt outstanding		$ 8,829,354			$ 8,805,574
Amount of debt authorized			$ 125,000
NSTAR Electric Company [Member]
Debt Instrument [Line Items]
Long-term debt outstanding		1,678,116		$ 700,000	1,829,766
The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Long-term debt outstanding		$ 2,516,010			$ 2,763,682
Subsequent Event [Member] | The Connecticut Light And Power Company [Member]
Debt Instrument [Line Items]
Amount of debt authorized	$ 1,325,000