Coal Trading (Details) - USD ($)		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net assets (liabilities) from coal trading activities		$ 0
Coal Trading [Member]
Trading Revenues by Type of Instrument [Line Items]
Total trading (losses) revenues		31,600,000	$ (8,800,000)
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets		157,500,000		$ 191,200,000
Liabilities from coal trading activities, gross amounts of recognized liabilities		(185,300,000)		(249,100,000)
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin		(27,800,000)		(57,900,000)
Gross amounts of coal trading liabilities offset against associated coal trading assets		(155,200,000)		(190,500,000)
Gross amounts of coal trading assets offset against associated coal trading liabilities		155,200,000		190,500,000
Net coal trading (liabilities) assets offset against associated assets (liabilities)		0		0
Variation margin held offset against assets from coal trading activities	[1]	(1,700,000)		0
Variation margin posted offset against liabilities from coal trading activities	[1]	29,400,000		57,400,000
Net variation margin (held) posted	[1]	27,700,000		57,400,000
Assets from coal trading activities, net		600,000		700,000
Liabilities from coal trading activities, net		(700,000)		(1,200,000)
Net assets (liabilities) from coal trading activities		(100,000)		(500,000)
Commodity futures, swaps and options [Member] | Coal Trading [Member]
Trading Revenues by Type of Instrument [Line Items]
Total trading (losses) revenues		7,100,000	(4,000,000)
Physical commodity purchase / sale contracts [Member] | Coal Trading [Member]
Trading Revenues by Type of Instrument [Line Items]
Total trading (losses) revenues		24,500,000	$ (4,800,000)
Cash Flow Hedging [Member] | Coal Trading [Member]
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net variation margin (held) posted		$ 0		$ 0

[1] None of the net variation margin (held) posted at March 31, 2017 and December 31, 2016, respectively, related to cash flow hedges.