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Guarantees (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012
Guarantees [Abstract]
Letters of credit posted in the Company's favor	$ 16.7
Guarantee obligations, maximum exposure, undiscounted	35.3
Excess of the current market cost of purchasing coal over the selling price	$ 70.0

The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity must commit resources to supply goods or services to one or more customers.

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