Industry Segment Results (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Sales-Outside	$ 836,128	$ 741,514	$ 1,706,394	$ 1,525,949
Natural Gas, NGLs and Oil Sales	229,743	171,236	496,041	339,078
Revenue from Purchased Oil and Gas	1,404	1,406	4,978	2,764
Royalty Interest Gas Revenue	18,335	17,028	44,980	31,232
Freight-Outside	10,109	9,660	20,054	21,913
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net	865,976	769,608	1,776,406	1,581,858
Total Revenue and Other Income	937,370	828,167	1,906,523	1,671,110
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(23,721)	38,127	106,459	33,511
Total Assets	11,566,647	12,752,928	11,566,647	12,752,928	11,393,667
Depreciation, Depletion and Amortization	137,899	109,529	267,015	221,107
Segment Reporting - Capital Expenditures	368,286	357,635	819,295	707,452
Income (Loss) from Equity Method Investments			(21,512)	(16,667)
XCoal Energy and Resources [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income		127,734	189,921	285,338
Duke Energy [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income	104,919	84,602	188,491	160,932
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	142,977	101,178	327,685	200,192
Total Assets	10,907,870	10,382,403	10,907,870	10,382,403
Gas
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	229,743	171,236	496,041	339,078
Revenue from Purchased Oil and Gas	1,404	1,406	4,978	2,764
Royalty Interest Gas Revenue	18,335	17,028	44,980	31,232
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	555	926	1,452	1,762
Revenue, Net		190,596	547,451	374,836
Total Revenue and Other Income	250,037
Income (Loss) from Continuing Operations before Income Taxes, Domestic	23,375	(4,607)	100,919	(5,250)
Total Assets	6,797,166	6,170,531	6,797,166	6,170,531
Depreciation, Depletion and Amortization	71,499	52,846	143,228	105,834
Segment Reporting - Capital Expenditures	304,486	188,464	570,456	395,593
Income (Loss) from Equity Method Investments	6,996	1,031	12,810	4,212
Equity Method Investments	259,870	170,589	259,870	170,589
Gas | Marcellus Shale [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	104,584	46,577	229,541	94,988
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		46,577	229,541	94,988
Total Revenue and Other Income	104,584
Income (Loss) from Continuing Operations before Income Taxes, Domestic	34,764	11,680	93,869	25,448
Gas | Coalbed Methane [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	80,681	87,799	176,752	171,439
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		87,799	176,752	171,439
Total Revenue and Other Income	80,681
Income (Loss) from Continuing Operations before Income Taxes, Domestic	17,949	22,125	51,568	43,305
Gas | Conventional Gas [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	26,556	33,745	58,901	66,181
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		33,745	58,901	66,181
Total Revenue and Other Income	26,556
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(5,096)	(5,591)	(6,853)	(9,629)
Gas | Other Gas
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	17,922	3,115	30,847	6,470
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	555	926	1,452	1,762
Revenue, Net		22,475	82,257	42,228
Total Revenue and Other Income	38,216
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(24,242)	(32,821)	(37,665)	(64,374)
Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside	536,298	505,060	1,070,979	1,052,969
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	10,109	9,660	20,054	21,913
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		514,720	1,091,033	1,074,882
Total Revenue and Other Income	546,407
Income (Loss) from Continuing Operations before Income Taxes, Domestic	119,602	105,785	226,766	205,442
Total Assets	4,110,704	4,211,872	4,110,704	4,211,872
Depreciation, Depletion and Amortization	65,086	55,247	121,149	112,437
Segment Reporting - Capital Expenditures	63,269	163,097	247,700	304,348
Income (Loss) from Equity Method Investments	6,933	10,762	9,793	12,294
Equity Method Investments	23,128	22,119	23,128	22,119
Coal Segment [Member] | Steam [Member]
Segment Reporting Information [Line Items]
Sales-Outside	446,416	335,926	863,385	681,866
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		335,926	863,385	681,866
Total Revenue and Other Income	446,416
Income (Loss) from Continuing Operations before Income Taxes, Domestic	111,906	87,915	260,473	181,373
Coal Segment [Member] | Low Volatile Metallurgical [Member]
Segment Reporting Information [Line Items]
Sales-Outside	66,771	111,006	151,312	257,834
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		111,006	151,312	257,834
Total Revenue and Other Income	66,771
Income (Loss) from Continuing Operations before Income Taxes, Domestic	9,292	30,818	20,722	85,535
Coal Segment [Member] | High Volatile Metallurgical [member]
Segment Reporting Information [Line Items]
Sales-Outside	20,483	53,189	49,415	102,667
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		53,189	49,415	102,667
Total Revenue and Other Income	20,483
Income (Loss) from Continuing Operations before Income Taxes, Domestic	7,571	16,768	16,675	27,506
Coal Segment [Member] | Other Coal [Member]
Segment Reporting Information [Line Items]
Sales-Outside	2,628	4,939	6,867	10,602
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	10,109	9,660	20,054	21,913
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		14,599	26,921	32,515
Total Revenue and Other Income	12,737
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(9,167)	(29,716)	(71,104)	(88,972)
Other Segments [Member]
Segment Reporting Information [Line Items]
Sales-Outside	70,087	65,218	139,374	133,902
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	20,034	32,427	39,346	67,905
Revenue, Net		97,645	178,720	201,807
Total Revenue and Other Income	90,121
Income (Loss) from Continuing Operations before Income Taxes, Domestic	1,755	4,887	3,252	7,461
Total Assets	297,044	363,819	297,044	363,819
Depreciation, Depletion and Amortization	1,314	1,436	2,638	2,836
Segment Reporting - Capital Expenditures	531	6,074	1,139	7,511
Income (Loss) from Equity Method Investments	133	76	(1,091)	160
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Sales-Outside	0	0	0	0
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	(20,589)	(33,353)	(40,798)	(69,667)
Revenue, Net		(33,353)	(40,798)	(69,667)
Total Revenue and Other Income	(20,589)
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(168,453)	(67,938)	(224,478)	(174,142)
Total Assets	361,733	2,006,706	361,733	2,006,706
Depreciation, Depletion and Amortization	0		0	0
Segment Reporting - Capital Expenditures	0		0	0
All Other
Segment Reporting Information [Line Items]
Equity Method Investments	$ 69,189	$ 63,389	$ 69,189	$ 63,389