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Coal Trading (Details 1) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Changes in the Company's recurring Level 3 net financial assets
Beginning of year		$ 281.2
Total gains (realized/unrealized):
End of year		295.1	$ 281.2
Coal Trading (Textuals) [Abstract]
Fair value hierarchy transfers from Level 1 to Level 2		0.0	0.0
Fair value hierarchy transfers from Level 2 to Level 1		0.0	0.0
Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Transfers Into Level 3		5.3	(4.4)	$ 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		(0.4)	0.0	0.0
Changes in the Company's recurring Level 3 net financial assets
Beginning of year		(15.6)	2.1	2.1
Total gains (realized/unrealized):
Included in earnings		(2.4)	(10.1)	6.7
Purchases		0.0	(0.5)	0.0
Sales		0.0	(0.1)	0.0
Settlements		12.0	(2.6)	(6.7)
End of year		(1.1)	(15.6)	2.1
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 (losses) gains	[1]	0.0	(6.2)	2.1
Coal Trading (Textuals) [Abstract]
Cash flow hedge derivative instrument assets at fair value		0.0
Fair value hierarchy transfers from Level 1 to Level 2		0.0	0.0	0.0
Fair value hierarchy transfers from Level 2 to Level 1		0.0	0.0	0.0
Fair Value hierarchy transfers out of Level 3			0.0	$ 0.0
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity futures, swaps and options		(0.1)	3.3
Physical commodity purchase/sale contracts		(0.4)	4.6
Total net financial assets (liabilities)		(0.5)	7.9
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity futures, swaps and options		0.0	0.0
Physical commodity purchase/sale contracts		0.0	0.0
Total net financial assets (liabilities)		0.0	0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity futures, swaps and options		(0.1)	3.3
Physical commodity purchase/sale contracts		0.7	20.2
Total net financial assets (liabilities)		0.6	23.5
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Coal Trading [Member]
Fair value coal trading net assets (liabilities) measured on recurring basis
Commodity futures, swaps and options		0.0	0.0
Physical commodity purchase/sale contracts		(1.1)	(15.6)
Total net financial assets (liabilities)		$ (1.1)	$ (15.6)
Minimum [Member] | Coal Trading [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation		2.00%
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation		26.00%
Maximum [Member] | Coal Trading [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation		2.00%
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation		26.00%
Weighted Average [Member] | Coal Trading [Member]
Coal Trading (Textuals) [Abstract]
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation		2.00%
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation		26.00%
[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.

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