Guarantees and Indemnifications (Details) (USD $) In Millions	Jun. 30, 2011
Guarantee Of Financial Obligations Of Npt Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	$ 18.8
Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2011 And 2012 Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	17.1
Guarantee Of Public Service Company Of New Hampshire And Select Energy Letters Of Credit Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	20.6
Guarantee Of Rocky River Reality And Nusco Lease Payments For Real Estate And Vehicles Expiring 2019 Through 2024 Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	21.8
Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	137.4
Guarantee Of Ngs Performance Expiring In 2020 Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	15.7
Guarantee Of Ngs Insurance Bond Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	1.2
Guarantee Of Es Boulos Surety Bonds Expiring Upon Project Completion Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	62.3
Guarantee Of Select Energy Wholesale Purchase Contracts Member
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	$ 58.2