REGULATORY ACCOUNTING - Regulatory Developments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Nov. 22, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 4,526.3		$ 4,583.8
Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	11.5
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] | Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	5.5
NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,347.1		1,461.4
NSTAR Electric Company [Member] | Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	3.2
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Regulatory assets	362.7		362.9
Public Service Company Of New Hampshire [Member] | Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	1.3
Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets	191.4		$ 191.2
Western Massachusetts Electric Company [Member] | Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Regulatory assets	1.5
FERC ROE First and Second Complaints [Member]
Regulatory Assets [Line Items]
Loss Contingency, Cumulative Pre-Tax Reserves Which Include Impact of Refunds Given to Customers	39.1
FERC ROE First and Second Complaints [Member] | The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Loss Contingency, Cumulative Pre-Tax Reserves Which Include Impact of Refunds Given to Customers	21.4
FERC ROE First and Second Complaints [Member] | NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Loss Contingency, Cumulative Pre-Tax Reserves Which Include Impact of Refunds Given to Customers	8.5
FERC ROE First and Second Complaints [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Loss Contingency, Cumulative Pre-Tax Reserves Which Include Impact of Refunds Given to Customers	3.1
FERC ROE First and Second Complaints [Member] | Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Loss Contingency, Cumulative Pre-Tax Reserves Which Include Impact of Refunds Given to Customers	6.1
Transmission Merger Cost Recovery [Member]
Regulatory Assets [Line Items]
Amount of recovery through transmission rates	27.5
Regulatory assets		$ 27.5
Transmission Merger Cost Recovery [Member] | The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Regulatory assets		13.2
Transmission Merger Cost Recovery [Member] | NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets		7.8
Transmission Merger Cost Recovery [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Regulatory assets		3.0
Transmission Merger Cost Recovery [Member] | Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Regulatory assets		$ 3.5
Yankee Companies [Member] | The Connecticut Light And Power Company [Member]
Regulatory Assets [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company	13.6
Yankee Companies [Member] | NSTAR Electric Company [Member]
Regulatory Assets [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company	5.0
Yankee Companies [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company	3.9
Yankee Companies [Member] | Western Massachusetts Electric Company [Member]
Regulatory Assets [Line Items]
Proceeds received from Yankee Companies to Eversource Utility Company	$ 3.6