Guarantees and Indemnifications (Details) (USD $) In Millions	Sep. 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	16.8
Guarantee Of Public Service Company Of New Hampshire And Select Energy Letters Of Credit [Member]
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	18.9
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	23.5
Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees [Member]
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	131.5
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	56.5
Guarantee Of Select Energy Wholesale Purchase Contracts [Member]
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	$ 58.1