Other Income, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Component Of Other Income Nonoperating [Line Items]
AFUDC Equity Funds	$ 18.8	$ 13.7	$ 7.1
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
AFUDC Equity Funds	5.2	2.9	2.9
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
AFUDC Equity Funds	1.2	0.6	0.2
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
AFUDC Equity Funds	$ 1.7	$ 1.7	$ 1.0