Guarantees and Indemnifications (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Guarantee Of Financial Obligations Of Npt [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$ 25.0
Various Subsidiary Surety Bonds Expiring Between 2012 And 2013 [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	35.0
Guarantee Of Letters Of Credit [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	19.5
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	21.9
Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	109.4
Guarantee Of Ngs Performance Expiring In 2020 [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	14.0
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	48.8
Guarantee Of Select Energy Wholesale Purchase Contracts [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	45.4
Guarantee of NE Hydro Companies' Long-Term Debt [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$ 6.6