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Commitments, Guarantees and Contingencies - Coal Rail and Shipping Contracts (Details) (Coal and Transportation [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2012
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Coal and Transportation [Member] |
Coal Rail and Shipping Contracts [Line Items] |
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2013 | $ 51.4
Coal Supply and Transportation Agreements, Minimum Annual Payment Obligation in 2014 | $ 0.8

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