Deposits (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Special Deposits [Line Items]
Cash Collateral Deposited	$ 17.1	$ 9.9
The Connecticut Light And Power Company [Member]
Special Deposits [Line Items]
Cash Collateral Deposited	0.7	1.2
NSTAR Electric Company [Member]
Special Deposits [Line Items]
Cash Collateral Deposited	8.5
Public Service Company Of New Hampshire [Member]
Special Deposits [Line Items]
Cash Collateral Deposited	$ 1.5	$ 2.5