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Leases (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Sale Leaseback Transaction (Numeric) [Line Items]      
Termination value of operating lease $ 6    
Percentage of leasehold interest in Colstrip Units 1 and 2 50.00%    
Percentage of leasehold interest in Colstrip Unit 3 30.00%    
Rent - Operating Leases (Numeric) [Abstract]      
Operating leases, rent expense 109 90 86
Operating leases, future minimum rental payments due (Details) [Abstract]      
2012 125    
2013 127    
2014 123    
2015 105    
2016 57    
Thereafter 252    
Total 789    
EW Brown Combustion Turbines [Member]
     
Sale Leaseback Transaction (Numeric) [Line Items]      
Number of assets contained in a sale-leaseback transaction 2    
Sale leaseback transaction date December 1999    
Term of leases related to sale-leaseback of generating units 18    
Number of years before early purchase option can be exercised 15.5    
Colstrip Generating Plant [Member]
     
Sale Leaseback Transaction (Numeric) [Line Items]      
Sale leaseback transaction date July 2000    
Term of leases related to sale-leaseback of generating units 36    
Termination value of operating lease $ 327    
Number of non-cancelable leases related to the sale-leaseback of generating units 4    
Number of renewal options for operating leases 2