Related Parties (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
The Connecticut Light And Power Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$ 25.0
NSTAR Electric Company [Member]
Related Party Transaction [Line Items]
Long Term Payable for Pension Payments Received From Affliates	70.2	75.9
Public Service Company Of New Hampshire [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	3.8
Western Massachusetts Electric Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$ 5.5