v2.4.0.6
Goodwill, transmission system rights, power purchase agreements and development intangible assets and liabilities (Details) (USD $)
 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Transmission system rights
Dec. 31, 2011
Northeast
Dec. 31, 2012
Northeast
Dec. 31, 2011
Northwest
Dec. 31, 2012
Northwest
Dec. 31, 2012
Southwest
Dec. 31, 2011
Southwest
Dec. 31, 2012
Un-allocated Corporate
Dec. 31, 2011
Un-allocated Corporate
Dec. 31, 2010
Un-allocated Corporate
Goodwill                        
Goodwill that is deductible for U.S. income tax purposes $ 43,700,000 $ 46,700,000                    
Changes in the carrying amount of goodwill                        
Balance at the beginning of the period 343,586,000 12,453,000     135,268,000   138,263,000 66,520,000 8,918,000 3,535,000 3,535,000 3,535,000
Acquisition of businesses   331,133,000   135,268,000   138,263,000     57,602,000      
Reclass to assets held for sale (8,918,000)             (8,918,000)        
Balance at the end of the period $ 334,668,000 $ 343,586,000   $ 135,268,000 $ 135,268,000 $ 138,263,000 $ 138,263,000 $ 57,602,000 $ 66,520,000 $ 3,535,000 $ 3,535,000 $ 3,535,000
Right to use percentage of Path 15 transmission line in California     72.00%