Guarantees and Indemnifications (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2009 Guarantee Of Financial Obligations Of Npt [Member]	Dec. 31, 2011 Various Subsidiary Surety Bonds And Performance Guarantees Expiring Between 2011 And 2012 [Member]	Dec. 31, 2011 Guarantee Of Public Service Company Of New Hampshire And Select Energy Letters Of Credit [Member]	Dec. 31, 2011 Guarantee Of Rocky River Reality And Nusco Lease Payments For Real Estate And Vehicles Expiring 2019 Through 2024 [Member]	Dec. 31, 2011 Guarantee Of Nuei Surety Bonds Insurance Bonds And Performance Guarantees [Member]	Dec. 31, 2011 Guarantee Of Ngs Performance Expiring In 2020 [Member]	Dec. 31, 2011 Guarantee Of Ngs Insurance Bond [Member]	Dec. 31, 2011 Guarantee Of Es Boulos Surety Bonds Expiring Upon Project Completion [Member]	Dec. 31, 2011 Guarantee Of Select Energy Wholesale Purchase Contracts [Member]
Guarantee Obligations Line Items
Guarantee Obligations Maximum Exposure	$ 18.8	$ 23.6	$ 17.9	$ 22.5	$ 92.1	$ 15.7	$ 1.2	$ 51.7	$ 23.5