Coal Trading (Details 1) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Mar. 31, 2015	Dec. 31, 2013
Coal Trading (Textuals) [Abstract]
Gain on discontinuation of cash flow hedge due to forecasted transaction no longer probable	$ 0.1
Level 1 to Level 2 transfers	0		0
Level 2 to Level 1 transfers	0		0
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
Coal Trading (Textuals) [Abstract]
Assets from coal trading activities designated as cash flow hedges at fair value before the application of variation margin	64.3					62.9
Expected amount of gains to be realized over the next 12 months in accumulated other comprehensive income					45
Level 1 to Level 2 transfers	0		0
Level 2 to Level 1 transfers	0		0
Level 3 transfers in and out, net	0		0
Changes in the Company's recurring Level 3 net financial assets
Beginning of period	2.1		5.2		2.2
Total net gains (losses) realized/unrealized:
Included in earnings	1.8		(4.1)
Settlements	(1.7)		0.3
End of period	2.2		1.4
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)	0.2	[1]	(3.2)	[1]
Estimated timing of future realization of coal trading portfolio by year of expiration:
2014	64.00%
2015	19.00%
2016	14.00%
2017	2.00%
2018	1.00%
Percentage of trading portfolio expiration, total	100.00%
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis:
Commodity futures, swaps and options	38.3					26.4
Physical commodity purchase/sale contracts	0.3					3.6
Total net financial assets	38.6					30.0
Level 1[Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis:
Commodity futures, swaps and options	0					0
Physical commodity purchase/sale contracts	0					0
Total net financial assets	0					0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis:
Commodity futures, swaps and options	38.3					26.4
Physical commodity purchase/sale contracts	(1.9)					1.5
Total net financial assets	36.4					27.9
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis:
Commodity futures, swaps and options	0					0
Physical commodity purchase/sale contracts	2.2					2.1
Total net financial assets	$ 2.2					$ 2.1
Minimum [Member] | Coal Trading [Member]
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%
Location differentials Level 3 unobservable input as percentage of overall valuation	1.00%
Maximum [Member] | Coal Trading [Member]
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	8.00%
Location differentials Level 3 unobservable input as percentage of overall valuation	27.00%
Weighted Average [Member] | Coal Trading [Member]
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	8.00%
Location differentials Level 3 unobservable input as percentage of overall valuation	25.00%

[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