Basic Service Bad Debt Adder (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Notes To Consolidated Financial Statements [Abstract]
Deferred Costs Associated With Bad Debt Adder At Nstar Electric	$ 34.0
Before Tax Reserve For Bad Debt Service Adder	28.0
After Tax Bad Debt Service Adder Reserve	$ 17.0

The amount of deferred costs through December 31, 2011 associated with the bad debt service adder at NSTAR Electric

No definition available.

Name:	nu_DeferredCostsAssociatedWithBadDebtAdderAtNstarElectric
Namespace Prefix:	nu_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant