INCOME TAXES - Tax Credits and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Federal Net Operating Loss		$ 15.5
Federal and State Tax Credit	$ 8.6	26.1
Federal and State Charitable Contribution	27.8	14.9
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	111.1	101.2
Federal and State Charitable Contribution	36.5	3.0
The Connecticut Light And Power Company [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Federal Net Operating Loss		0.0
Federal and State Tax Credit	0.0	0.1
Federal and State Charitable Contribution	0.0	0.0
The Connecticut Light And Power Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	80.5	73.8
Federal and State Charitable Contribution	0.0	0.0
NSTAR Electric Company [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Federal Net Operating Loss		0.0
Federal and State Tax Credit	0.0	0.2
Federal and State Charitable Contribution	0.0	0.0
NSTAR Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
Public Service Company Of New Hampshire [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Federal Net Operating Loss		7.0
Federal and State Tax Credit	0.0	15.0
Federal and State Charitable Contribution	0.0	0.0
Public Service Company Of New Hampshire [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
Western Massachusetts Electric Company [Member] | Internal Revenue Service (IRS) [Member]
Operating Loss Carryforwards [Line Items]
Federal Net Operating Loss		1.0
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	0.0	0.0
Western Massachusetts Electric Company [Member] | State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Federal and State Tax Credit	0.0	0.0
Federal and State Charitable Contribution	$ 0.0	$ 0.0