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Commitments, Guarantees and Contingencies - Coal, Rail and Shipping Contracts (Details) (Coal and Transportation [Member], USD $)
In Millions, unless otherwise specified
Mar. 31, 2014
Coal and Transportation [Member]
 
Coal, Rail and Shipping Contracts [Line Items]  
Minimum Annual Payment Obligation, Current Year $ 26.0
Minimum Annual Payment Obligation, Year Two $ 4.0