REGULATORY ACCOUNTING - Equity Return on Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 1.0	$ 1.2
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	42.0	$ 44.9
Clean Air Project | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 25.0