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Derivative Instruments (Details 1) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2011
Dec. 31, 2010
Net forward notional buy (sell) line items [Abstract]  
Nonmonetary notional amounts - Power (MWh)(38,000,000)(50,000,000)
Nonmonetary notional amounts - Natural Gas (MMBtu)124,000,00031,000,000
Interest rate swaps$ 5,448[1]$ 6,171[1]
[1]

Approximately $4.1 billion and $3.3 billion at September 30, 2011 and December 31, 2010, respectively, related to variable rate debt that was converted to fixed rate debt in 2011 and 2010.