Coal Trading (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 30, 2016
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net assets (liabilities) from coal trading activities					$ 0
Coal Trading [Member]
Trading revenue
Trading revenue		$ (10,900,000)	$ 42,800,000	$ 58,400,000
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets		191,200,000	128,600,000
Liabilities from coal trading activities, gross amounts of recognized liabilities		(249,100,000)	(110,000,000)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin		(57,900,000)	18,600,000
Gross amounts of coal trading liabilities offset against associated coal trading assets		(190,500,000)	(87,300,000)
Gross amounts of coal trading assets offset against associated coal trading liabilities		190,500,000	87,300,000
Net coal trading assets (liabilities) offset against associated (liabilities) assets		0	0
Variation margin held offset against assets from coal trading activities		0	(17,800,000)
Variation margin posted offset against liabilities from coal trading activities		57,400,000	7,100,000
Net variation margin (held) posted	[1]	57,400,000	10,700,000
Assets from coal trading activities, net		700,000	23,500,000
Liabilities from coal trading activities, net		(1,200,000)	(15,600,000)
Net assets (liabilities) from coal trading activities		(500,000)	7,900,000
Commodity futures, swaps and options [Member] | Coal Trading [Member]
Trading revenue
Trading revenue		(96,500,000)	107,300,000	92,300,000
Physical commodity purchase / sale contracts [Member] | Coal Trading [Member]
Trading revenue
Trading revenue		$ 85,600,000	(64,500,000)	$ (33,900,000)
Cash Flow Hedging [Member] | Coal Trading [Member]
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net variation margin (held) posted			$ 0

[1] None of the net variation margin (held) posted at December 31, 2016 and 2015, respectively, related to cash flow hedges.