Coal Trading (Details 1) (USD $) In Millions, unless otherwise specified			3 Months Ended				9 Months Ended													3 Months Ended			12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Coal Trading [Member]		Sep. 30, 2013 Coal Trading [Member]		Sep. 30, 2014 Coal Trading [Member]		Sep. 30, 2013 Coal Trading [Member]		Dec. 31, 2013 Coal Trading [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Sep. 30, 2014 Level 1 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Dec. 31, 2013 Level 1 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Sep. 30, 2014 Level 2 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Dec. 31, 2013 Level 2 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Sep. 30, 2014 Level 3 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Dec. 31, 2013 Level 3 [Member] Fair Value, Measurements, Recurring [Member] Coal Trading [Member]	Sep. 30, 2014 Minimum [Member] Coal Trading [Member]	Sep. 30, 2014 Maximum [Member] Coal Trading [Member]	Sep. 30, 2014 Weighted Average [Member] Coal Trading [Member]	Sep. 30, 2015 Scenario, Forecast [Member] Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimate of Time to Transfer																							12 months
Fair value coal trading net assets (liabilities) measured on recurring basis:
Commodity futures, swaps and options												$ 35.2	$ 26.4	$ 0	$ 0	$ 35.2	$ 26.4	$ 0	$ 0
Physical commodity purchase/sale contracts												(4.9)	3.6	0	0	(7.2)	1.5	2.3	2.1
Total net financial assets												30.3	30.0	0	0	28.0	27.9	2.3	2.1
Coal Trading (Textuals) [Abstract]
Assets from coal trading activities designated as cash flow hedges at fair value before the application of variation margin			45.8				45.8				62.9
Expected amount of gains to be realized over the next 12 months in accumulated other comprehensive income																							40
Gain on discontinuation of cash flow hedge due to forecasted transaction no longer probable							2.0
Level 1 to Level 2 transfers	0	0	0		0		0		0
Level 2 to Level 1 transfers	0	0	0		0		0		0
Level 3 transfers in and out, net			0		0		0		0
Summary of quantitative unobservable inputs related to coal trading Level 3 fair value measurements:
Quality adjustment Level 3 unobservable input as percentage of overall valuation																				1.00%	9.00%	7.00%
Location differentials Level 3 unobservable input as percentage of overall valuation																				18.00%	66.00%	29.00%
Changes in the Company's recurring Level 3 net financial assets
Beginning of period			2.6		1.3		2.1		5.2
Total net gains (losses) realized/unrealized:
Included in earnings			2.0		(2.7)		5.3		(7.1)
Settlements			(2.3)		1.8		(5.1)		2.3
End of period			2.3		0.4		2.3		0.4
Changes in net unrealized gains (losses) relating to Level 3 net financial assets held both as of the beginning and the end of the period:
Changes in net unrealized gains (losses)			$ 1.1	[1]	$ (3.0)	[1]	$ 1.6	[1]	$ (2.9)	[1]
Estimated timing of future realization of coal trading portfolio by year of expiration:
2014			25.00%				25.00%
2015			56.00%				56.00%
2016			14.00%				14.00%
2017			3.00%				3.00%
2018			2.00%				2.00%
Percentage of trading portfolio expiration, total			100.00%				100.00%

[1] Within the unaudited condensed consolidated statements of operations 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