Reconciliation of Segment Information, Revenue and Other Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	$ 1,125,776	$ 1,189,293	$ 2,351,941	$ 2,500,764
Earnings Before Income Taxes:
Earnings Before Income Taxes	1,809	211,655	510	330,232
Revenue from Purchased Oil and Gas	1,406	651	2,764	1,490
Sales-Gas Royalty Interests	17,028	9,533	31,232	21,739
Freight-Outside	10,125	49,472	24,186	98,765
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	1,154,335	1,248,949	2,410,123	2,622,758
Total Assets	12,752,928	12,597,896	12,752,928	12,597,896	12,670,909
Depreciation, Depletion and Amortization	159,307	153,824	320,622	309,171
Capital Expenditures	352,028	407,953	758,000	714,399
Total Reportable Business Segments
Earnings Before Income Taxes:
Earnings Before Income Taxes	60,868	258,325	153,730	433,295
Total Assets	11,771,465	11,416,441	11,771,465	11,416,441
Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	871,071	944,770	1,843,456	1,999,086
Earnings Before Income Taxes:
Earnings Before Income Taxes	65,478	257,181	158,980	419,651
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	10,125	49,472	24,186	98,765
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	881,196	994,242	1,867,642	2,097,851
Total Assets	5,600,934	5,445,502	5,600,934	5,445,502
Depreciation, Depletion and Amortization	100,751	100,684	203,462	201,446
Capital Expenditures	157,558	253,587	354,896	448,016
Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	171,236	148,099	339,078	308,302
Earnings Before Income Taxes:
Earnings Before Income Taxes	(4,610)	1,144	(5,250)	13,644
Revenue from Purchased Oil and Gas	1,406	651	2,764	1,490
Sales-Gas Royalty Interests	17,028	9,533	31,232	21,739
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	926	360	1,762	826
Segment Reporting Information, Revenue for Reportable Segment	190,596	158,643	374,836	332,357
Total Assets	6,170,531	5,970,939	6,170,531	5,970,939
Depreciation, Depletion and Amortization	52,236	47,326	104,635	96,129
Capital Expenditures	188,463	143,206	395,593	241,661
All Other Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	83,469	96,424	169,407	193,376
Earnings Before Income Taxes:
Earnings Before Income Taxes	(883)	14,962	(41,435)	19,045
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	32,428	36,136	67,905	73,345
Segment Reporting Information, Revenue for Reportable Segment	115,897	132,560	237,312	266,721
Total Assets	363,819	360,673	363,819	360,673
Depreciation, Depletion and Amortization	6,320	(5,782)	12,525	0
Capital Expenditures	6,007	11,160	7,511	24,722
Excludes Gas Segment [Member]
Earnings Before Income Taxes:
Interest Income (Expense), Net and Other Non-operating Activity	(56,406)	(58,943)	(109,066)	(118,985)
Fees for Disposing Non-core Assets	(1,770)	(2,689)	2,719	3,123
Intersegment Elimination [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	0	0	0	0
Earnings Before Income Taxes:
Earnings Before Income Taxes	(58,176)	(61,632)	(111,785)	(122,108)
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	(33,354)	(36,496)	(69,667)	(74,171)
Segment Reporting Information, Revenue for Reportable Segment	(33,354)	(36,496)	(69,667)	(74,171)
Total Assets	617,644	820,782	617,644	820,782
Depreciation, Depletion and Amortization	0	11,596	0	11,596
Capital Expenditures	0	0	0	0
Steam [Member] \| Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	697,945	748,303	1,459,217	1,560,356
Earnings Before Income Taxes:
Earnings Before Income Taxes	103,768	133,697	232,200	262,146
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	697,945	748,303	1,459,217	1,560,356
High Volatile Metallurgical [member] \| Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	57,115	71,250	115,737	131,818
Earnings Before Income Taxes:
Earnings Before Income Taxes	17,245	19,418	30,597	35,354
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	57,115	71,250	115,737	131,818
Low Volatile Metallurgical [Member] \| Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	111,006	120,481	257,834	293,221
Earnings Before Income Taxes:
Earnings Before Income Taxes	30,819	42,780	85,536	122,121
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	111,006	120,481	257,834	293,221
Other Coal [Member] \| Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	5,005	4,736	10,668	13,691
Earnings Before Income Taxes:
Earnings Before Income Taxes	(86,354)	61,286	(189,353)	30
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	10,125	49,472	24,186	98,765
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	15,130	54,208	34,854	112,456
Coalbed Methane [Member] \| Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	87,799	88,080	171,439	187,615
Earnings Before Income Taxes:
Earnings Before Income Taxes	22,124	24,344	43,436	60,734
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	87,799	88,080	171,439	187,615
Marcellus Shale [Member] \| Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	46,577	23,730	94,988	47,521
Earnings Before Income Taxes:
Earnings Before Income Taxes	11,680	4,835	25,448	8,086
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	46,577	23,730	94,988	47,521
Conventional Gas [Member] \| Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	33,745	34,207	66,181	68,580
Earnings Before Income Taxes:
Earnings Before Income Taxes	(5,576)	(2,410)	(9,737)	(6,132)
Revenue from Purchased Oil and Gas	0	0	0	0
Sales-Gas Royalty Interests	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	0	0	0	0
Segment Reporting Information, Revenue for Reportable Segment	33,745	34,207	66,181	68,580
Other Gas \| Gas
Segment Reporting, Revenue Reconciling Item [Line Items]
Sales-Outside	3,115	2,082	6,470	4,586
Earnings Before Income Taxes:
Earnings Before Income Taxes	(32,838)	(25,625)	(64,397)	(49,044)
Revenue from Purchased Oil and Gas	1,406	651	2,764	1,490
Sales-Gas Royalty Interests	17,028	9,533	31,232	21,739
Freight-Outside	0	0	0	0
Segment Reporting Information, Intersegment Revenue	926	360	1,762	826
Segment Reporting Information, Revenue for Reportable Segment	22,475	12,626	42,228	28,641
First Energy [Member] \| Coal
Segment Reporting, Revenue Reconciling Item [Line Items]
Entity-Wide Revenue, Major Customer, Amount	$ 157,318	$ 136,576	$ 328,300	$ 280,731

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