Guarantees (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 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	22.3
Performance Guarantee [Member] | Magnum [Member]
Volume of purchased coal in tons	900,000
Magnum [Member]
Guarantee obligations, maximum exposure, undiscounted	19.5
Excess of the current market cost of purchasing coal over the selling price	$ 264.3
Volume of purchased coal in tons	10,300,000