Related Parties (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
The Connecticut Light And Power Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$ 25.0	$ 25.0
Public Service Company Of New Hampshire [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	3.8	3.8
Western Massachusetts Electric Company [Member]
Related Party Transaction [Line Items]
Long Term Receivable From Nusco	$ 5.5	$ 5.5