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Commitments, Guarantees and Contingencies - Coal Rail and Shipping Contracts (Details) (Public Utilities, Inventory, Coal [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Public Utilities, Inventory, Coal [Member]
 
Coal Rail and Shipping Contracts [Line Items]  
Recorded Unconditional Purchase Obligation Due within One Year $ 55.4
Recorded Unconditional Purchase Obligation Due in Second Year $ 27.0