Allowance for Funds Used During Construction (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	$ 4.1	$ 5.3	$ 11.8
Public Utilities Allowance For Funds Used During Construction Additions	7.1	6.8	22.5
Public Utilities Allowance For Funds Used During Construction Capitalized Total	11.2	12.1	34.3
Public Utilities Allowance For Funds Used During Construction Rate	2.70%	3.70%	7.30%
The Connecticut Light And Power Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	2.2	2.5	3.3
Public Utilities Allowance For Funds Used During Construction Additions	2.9	1.9	6.0
Public Utilities Allowance For Funds Used During Construction Capitalized Total	5.1	4.4	9.3
Public Utilities Allowance For Funds Used During Construction Rate	3.70%	3.60%	8.30%
NSTAR Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.5	0.3	0.2
Public Utilities Allowance For Funds Used During Construction Capitalized Total	0.5	0.3	0.2
Public Utilities Allowance For Funds Used During Construction Rate	0.50%	0.40%	0.30%
Public Service Company Of New Hampshire [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.5	1.6	7.1
Public Utilities Allowance For Funds Used During Construction Additions	0.2	1.9	13.2
Public Utilities Allowance For Funds Used During Construction Capitalized Total	0.7	3.5	20.3
Public Utilities Allowance For Funds Used During Construction Rate	1.10%	5.90%	7.10%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
Public Utilities Allowance For Funds Used During Construction Capitalized Interest	0.5	0.5	0.5
Public Utilities Allowance For Funds Used During Construction Additions	1.0	1.0	1.0
Public Utilities Allowance For Funds Used During Construction Capitalized Total	$ 1.5	$ 1.5	$ 1.5
Public Utilities Allowance For Funds Used During Construction Rate	6.10%	6.80%	7.40%