REGULATORY ACCOUNTING - Narrative (Details) - USD ($) $ in Millions	Jun. 30, 2018	May 08, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 5,396.3		$ 5,239.3
Amount of regulatory costs not yet approved	124.4		105.8
Regulatory liabilities	3,937.1		3,765.4
Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	989.2		940.8
Amount of regulatory costs not yet approved	30.2		27.2
Regulatory liabilities	477.9		450.7
The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory assets	1,755.2		1,645.2
Amount of regulatory costs not yet approved	24.1		18.2
Regulatory liabilities	1,196.9		1,151.1
NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	1,515.7		1,524.5
Amount of regulatory costs not yet approved	49.8		42.7
Regulatory liabilities	1,631.0		1,594.1
Deferred Costs from Generation Asset Sale
Regulatory Assets [Line Items]
Regulatory assets	274.0
Deferred Costs from Generation Asset Sale | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory assets	16.0
Deferred Costs from Generation Asset Sale | The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory assets	142.0
Deferred Costs from Generation Asset Sale | NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory assets	116.0
Rate Reduction Bonds | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Amount of securitized rate reduction bonds issued		$ 635.7
Revenue Subject to Refund
Regulatory Assets [Line Items]
Regulatory liabilities	36.5		0.0
Revenue Subject to Refund | Public Service Company of New Hampshire
Regulatory Assets [Line Items]
Regulatory liabilities	6.2		0.0
Revenue Subject to Refund | The Connecticut Light and Power Company
Regulatory Assets [Line Items]
Regulatory liabilities	16.6		0.0
Revenue Subject to Refund | NSTAR Electric Company
Regulatory Assets [Line Items]
Regulatory liabilities	$ 3.7		$ 0.0