Deferred Contractual Obligations & Other (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Loss Contingency, Information about Litigation Matters [Abstract]
Court of Federal Claims Ruling To CYAPC	$ 34.2
Court Of Federal Claims Ruling To YAEC	32.9
Court Of Federal Claims Ruling To MYAPC	75.8
Trial Court Ruling To CYAPC	39.7
Trial Court Ruling To YAEC	21.2
Trial Court Ruling To MYAPC	81.7
Receivable from DOE at Year End 2012	77.9
Connecticut Yankee Proceeds Received	39.6
Yankee Atomic Proceeds Received	38.3
Maine Yankee Proceeds Received	81.7
NSTAR Electric DPU Storm Penalty	4.1
WMECO DPU Storm Penalty	2.0
Cumulative CPSL Costs 2006 To 2011	83.0
CPSL Revenue Requirement For 2012 To 2015	15.0
Bad Debt Service Adder As Of 2011	34.0
Pre Tax NSTAR Electric Bad Debt Adder Reserve Recorded In 2012	28.0
Post Tax NSTAR Electric Bad Debt Adder Reserve In 2012	$ 17.0