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Guarantees (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012
Surety bonds posted in the company's favor	$ 48.9
Letters of credit posted in the Company's favor	16.7
Guarantee obligations, maximum exposure, undiscounted	38.5
Volume of purchased coal in tons	573
Performance Guarantee [Member] \| Magnum [Member]
Guarantee obligations, maximum exposure, undiscounted	64.1
Excess of the current market cost of purchasing coal over the selling price	12.2
Indemnification Agreement [Member]
Guarantee obligations, maximum exposure, undiscounted	$ 16.3

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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