Coal Trading (Details 1) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		$ 204.4		$ 324.4
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
Schedule of future realization of trading portfolio
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(78.7)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		16.3
Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value		3.9		15.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			$ 2.2		$ 2.1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3		(10.7)
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss)	[1]	(0.1)	0.5
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
Schedule of future realization of trading portfolio
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers out of Level 3			0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		0.1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings			0.5
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Sales		(0.1)	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		1.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases, (Sales), Issuances, (Settlements)			$ (0.4)
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Futures, Swaps And Options Fair Value Disclosure		1.3		3.3
Physical Purchase Sale Contracts Fair Value Disclosure		0.6		4.6
Fair value coal trading net assets (liabilities) measured on recurring basis:
Total net financial (liabilities) assets		1.9		7.9
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Futures, Swaps And Options Fair Value Disclosure		0.0		0.0
Physical Purchase Sale Contracts Fair Value Disclosure		0.0		0.0
Fair value coal trading net assets (liabilities) measured on recurring basis:
Total net financial (liabilities) assets		0.0		0.0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Futures, Swaps And Options Fair Value Disclosure		1.3		3.3
Physical Purchase Sale Contracts Fair Value Disclosure		4.5		20.2
Fair value coal trading net assets (liabilities) measured on recurring basis:
Total net financial (liabilities) assets		5.8		23.5
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Coal Trading [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Futures, Swaps And Options Fair Value Disclosure		0.0		0.0
Physical Purchase Sale Contracts Fair Value Disclosure		(3.9)		(15.6)
Fair value coal trading net assets (liabilities) measured on recurring basis:
Total net financial (liabilities) assets		$ (3.9)		$ (15.6)

[1] (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.