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Guarantees (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
Bonding Requirements [Member] \| Magnum [Member]
Surety bonds posted in the company's favor	$ 48.9
Letters of credit posted in the Company's favor	32.7
Guarantee obligations, maximum exposure, undiscounted	38.5
Indemnification Agreement [Member]
Guarantee obligations, maximum exposure, undiscounted	25.3
Performance Guarantee [Member] \| Magnum [Member]
Volume of purchased coal in tons	1,100,000
Magnum [Member]
Guarantee obligations, maximum exposure, undiscounted	23.8
Excess of the current market cost of purchasing coal over the selling price	$ 368.2
Volume of purchased coal in tons	10,700,000

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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