Guarantees and Indemnifications (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Guarantee Of Financial Obligations Of Npt [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$ 25.0
Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2013 And 2015 [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	32.8
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	20.1
Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	67.4
Guarantee Of Ngs Insurance Bond [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	1.0
Guarantee Of Es Boulos Surety Bonds Expiring Upon Project Completion [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	52.9
Guarantee Of Select Energy Wholesale Purchase Contracts [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	13.5
Guarantee Of Debt Of Ne Hydros [Member]
Guarantee Obligations [Line Items]
Guarantee Obligations Maximum Exposure	$ 5.6