Proposed Merger with NSTAR (Details) In Millions, unless otherwise specified	Dec. 31, 2011
Business Acquisition [Line Items]
Exchange Ratio For Common Shares Converted As A Result Of Merger	1.312
Common Shares Issued As A Result Of Merger	137.0
NSTAR Ownership Post Merger	44.00%
NU Ownership Post Merger	56.00%

Represents exhange ratio under the NU/NSTAR merger agreement that NSTAR shareholders will receive for each share of NU common shares

No definition available.

Name:	nu_ExchangeRatioForCommonSharesConvertedAsAResultOfMerger
Namespace Prefix:	nu_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant