REGULATORY ACCOUNTING - Storm Restoration Costs (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory assets	$ 5,239.3	$ 4,526.3
The Connecticut Light And Power Company
Regulatory Assets [Line Items]
Regulatory assets	1,645.2	1,727.1
NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	1,524.5	1,538.5
Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	$ 940.8	$ 362.7