REGULATORY ACCOUNTING - Equity Return on Regulatory Assets (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 1.2	$ 1.5
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	44.9	$ 48.3
Clean Air Project [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Equity return not recorded on the balance sheet	$ 25.0