REGULATORY ACCOUNTING - Equity Return on Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 0.7	$ 1.0
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 12.0	42.0
Clean Air Project | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet		$ 25.0