Deferred Contractual Obligations & Other (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Loss Contingency, Information about Litigation Matters [Abstract]
Trial Court Ruling To CYAPC	$ 39.7
Trial Court Ruling To YAEC	38.2
Trial Court Ruling To MYAPC	81.7
Increase In Marketable Securities Current Period	77.9
Connecticut Yankee Proceeds Received	39.7
Yankee Atomic Proceeds Received	38.2
Maine Yankee Proceeds Received	81.7
Cumulative CPSL Costs 2006 To 2011	83.0
Bad Debt Service Adder As Of 2011	$ 34.0