Proposed Merger with NSTAR (Details) In Millions, unless otherwise specified	Sep. 30, 2011
Notes To Consolidated Financial Statements [Abstract]
Exchange Ratio For Common Shares Converted As A Result Of Merger	1.312
NU Ownership Post Merger	56.00%
NSTAR Ownership Post Merger	44.00%
Common Shares Issued As A Result Of Merger	137.0