REGULATORY ACCOUNTING - Storm Restoration Costs (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 4,526.3	$ 4,583.8
Storm Restoration Costs Pending Approval [Member]
Regulatory Assets [Line Items]
Regulatory assets	239.0
The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,727.1	1,637.3
The Connecticut Light And Power Company [Member] | Storm Restoration Costs Pending Approval [Member]
Regulatory Assets [Line Items]
Regulatory assets	33.0
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,347.1	1,461.4
NSTAR Electric Company [Member] | Storm Restoration Costs Pending Approval [Member]
Regulatory Assets [Line Items]
Regulatory assets	124.0
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Regulatory assets	362.7	362.9
Public Service Company Of New Hampshire [Member] | Storm Restoration Costs Pending Approval [Member]
Regulatory Assets [Line Items]
Regulatory assets	78.0
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets	191.4	$ 191.2
Western Massachusetts Electric Company [Member] | Storm Restoration Costs Pending Approval [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 4.0