Other Income, Net (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$ 13.7	$ 7.1	$ 6.8
The Connecticut Light And Power Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	2.9	2.9	1.9
Public Service Company Of New Hampshire [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	0.6	0.2	1.9
Western Massachusetts Electric Company [Member]
Component Of Other Income Nonoperating [Line Items]
Public Utilities Allowance For Funds Used During Construction Additions	$ 1.7	$ 1.0	$ 1.0