Coal Trading (Details 1) (USD $)	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2011
Coal Trading (Textuals) [Abstract]
Margin posted to counterparties related to credit rating	$ 0
Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Quality Adjustment Level 3 Unobservable Input as a percentage of overall valuation, Lower Range	5.00%
Fair value coal trading net assets (liabilities) measured on recurring basis
Cash Flow Hedge Derivative Instrument Assets at Fair Value	48,800,000				22,400,000
Changes in the Company's recurring Level 3 net financial assets (liabilities)
Beginning of period	8,700,000		18,600,000
Total gains or losses (realized/unrealized):
Included in earnings	1,500,000		10,100,000
Included in other comprehensive income	0		0
Settlements	(1,300,000)		3,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0
Transfers out	0		(19,200,000)
End of period	8,900,000		12,500,000
Changes in unrealized gains (losses) relating to Level 3 net financial assets held both at the beginning and the end of the period
Changes in unrealized gains (losses)	1,900,000	[1]	10,000,000	[1]
Schedule of future realization of trading portfolio
2012	59.00%
2013	32.00%
2014	6.00%
2015	3.00%
Percentage of trading portfolio expiration, total	100.00%
Coal Trading (Textuals) [Abstract]
Margin posted with counterparties and exchanges	(71,600,000)				(52,100,000)
Net Margin Posted With Counterparties For Cash Flow Hedge	(50,000,000)				(23,000,000)
Expected amount of gains to be realized over the next 12 months in accumulated other comprehensive income	32,000,000
Estimated time of transfer (in months)	12 months
Potential collateralization that may be requested by counterparties related to material adverse event	15,000,000				11,000,000
Margin posted to counterparties related to material adverse event	0		0		0
Additional potential collateral requirements for a credit downgrade	0				0
Variation margin posted	(71,600,000)				(52,100,000)
Initial margin posted	28,000,000				34,000,000
Margin in excess of the exchange-required variation and initial margin	(18,300,000)
Losses reclassified to earnings as forecasted transactions no longer probable	1,200,000
Funds Received from MF Global	20,000,000
Funds Remaining with MF Global	52,600,000
Quality Adjustment Level 3 Unobservable Input as percentage of overall valuation, Higher Range	20.00%
Quality Adjustment Level 3 Unobservable Input as a percentage of overall valuation, Weighted Average	11.00%
Location Differential Level 3 Unobservable Input as a percentage of overall valuation, Lower Range	20.00%
Location Differential Level 3 Unobservable Input as a percentage of overall valuation, Higher Range	45.00%
Location Differential Level 3 Unobservable Input as a percentage of overall valuation, Weighted Average	24.00%
Nonperformance Adjustment Level 3 Unobservable Input as percentage of overall valuation	4.00%
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	16,000,000				19,300,000
Physical commodity purchase/sale contracts	12,700,000				15,000,000
Total net financial assets (liabilities)	28,700,000				34,300,000
Level 1[Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	5,000,000				21,200,000
Physical commodity purchase/sale contracts	0				0
Total net financial assets (liabilities)	5,000,000				21,200,000
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	11,000,000				(1,900,000)
Physical commodity purchase/sale contracts	3,800,000				6,300,000
Total net financial assets (liabilities)	14,800,000				4,400,000
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	0				0
Physical commodity purchase/sale contracts	8,900,000				8,700,000
Total net financial assets (liabilities)	$ 8,900,000				$ 8,700,000

[1] Within the unaudited condensed consolidated statements of income and unaudited condensed consolidated statements of comprehensive income for the periods presented, unrealized gains and losses from Level 3 items are combined with unrealized gains and losses on positions classified in Level 1 or 2, as well as other positions that have been realized during the applicable periods