Allowance for Funds Used During Construction (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance For Funds Used During Construction [Line Items]
AFUDC Borrowed Funds	$ 7.2	$ 5.8	$ 4.1
AFUDC Equity Funds	18.8	13.7	7.1
Total AFUDC	$ 26.0	$ 19.5	$ 11.2
Average AFUDC Rate	3.90%	3.40%	2.70%
The Connecticut Light And Power Company [Member]
Allowance For Funds Used During Construction [Line Items]
AFUDC Borrowed Funds	$ 2.6	$ 1.9	$ 2.2
AFUDC Equity Funds	5.2	2.9	2.9
Total AFUDC	$ 7.8	$ 4.8	$ 5.1
Average AFUDC Rate	5.50%	3.40%	3.70%
NSTAR Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
AFUDC Borrowed Funds	$ 2.0	$ 2.0	$ 0.5
AFUDC Equity Funds	4.3	3.8
Total AFUDC	$ 6.3	$ 5.8	$ 0.5
Average AFUDC Rate	3.20%	2.50%	0.50%
Public Service Company Of New Hampshire [Member]
Allowance For Funds Used During Construction [Line Items]
AFUDC Borrowed Funds	$ 1.0	$ 0.6	$ 0.5
AFUDC Equity Funds	1.2	0.6	0.2
Total AFUDC	$ 2.2	$ 1.2	$ 0.7
Average AFUDC Rate	1.80%	1.80%	1.10%
Western Massachusetts Electric Company [Member]
Allowance For Funds Used During Construction [Line Items]
AFUDC Borrowed Funds	$ 1.0	$ 0.9	$ 0.5
AFUDC Equity Funds	1.7	1.7	1.0
Total AFUDC	$ 2.7	$ 2.6	$ 1.5
Average AFUDC Rate	4.40%	5.60%	6.10%