Coal Trading (Details 1) (Coal Trading [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Changes in the Company's recurring Level 3 net financial assets (liabilities)
Beginning of period	$ 8.7		$ 18.6		$ 17.0
Total gains or losses (realized/unrealized):
Included in earnings	17.5		8.9		2.1
Included in other comprehensive income	0		0		(0.5)
Settlements	(21.0)		(2.1)		(0.1)
Transfers in	0		0		1.0
Transfers out	0		(17.7)		0.1
End of period	5.2		8.7		18.6
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	4.1	[1]	8.7	[1]	6.7	[1]
Schedule of future realization of trading portfolio
2013	77.00%
2014	15.00%
2015	6.00%
2016	2.00%
Percentage of trading portfolio expiration, total	100.00%
Coal Trading (Textuals) [Abstract]
Gain on discontinuation of cash glow hedge due to forecasted transaction probable of not occurring	7.5
Gain on discontinuation of cash flow hedge due to forecasted transaction probable of not occurring from counterparty bankruptcy	4.5
Cash flow hedge derivative instrument assets at fair value	153.1		22.4
Fair value hierarchy transfers from Level 1 to Level 2	0		0		0
Fair value hierarchy transfers from Level 2 to Level 1	0		0		0
Estimated time of transfer	12 months
Expected amount of gains to be realized over the next 12 months in accumulated other comprehensive income	132
Fair Value, Measurements, Recurring [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	25.6		19.3
Physical commodity purchase/sale contracts	7.4		15.0
Total net financial assets	33.0		34.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	1.2		21.2
Physical commodity purchase/sale contracts	0		0
Total net financial assets	1.2		21.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	24.4		(1.9)
Physical commodity purchase/sale contracts	2.2		6.3
Total net financial assets	26.6		4.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity swaps and options	0		0
Physical commodity purchase/sale contracts	5.2		8.7
Total net financial assets	$ 5.2		$ 8.7
Minimum [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation	2.00%
Nonperformance adjustment Level 3 unobservable inputs as percentage of overall valuation	4.00%
Maximum [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation	22.00%
Nonperformance adjustment Level 3 unobservable inputs as percentage of overall valuation	4.00%
Weighted Average [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation	14.00%
Nonperformance adjustment Level 3 unobservable inputs as percentage of overall valuation	4.00%

[1] Within the consolidated statements of operations and 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.