Industry Segment Results (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Sales-Outside	$ 814,991	$ 735,968	$ 2,521,385	$ 2,261,917
Natural Gas, NGLs and Oil Sales	257,358	192,781	753,399	531,859
Revenue from Purchased Oil and Gas	1,205	1,607	6,183	4,372
Royalty Interest Gas Revenue	17,610	15,506	62,590	46,737
Freight-Outside	2,497	9,579	22,551	31,492
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net	836,303	762,660	2,612,709	2,344,518
Total Revenue and Other Income	884,616	803,345	2,791,139	2,474,455
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(3,033)	(3,311)	103,426	30,200
Total Assets	11,718,935	12,737,072	11,718,935	12,737,072	11,393,667
Depreciation, Depletion and Amortization	148,665	117,730	415,680	338,837
Segment Reporting - Capital Expenditures	355,312	313,675	1,174,607	1,021,127
Equity in Earnings of Affiliates			(38,477)	(20,276)
XCoal Energy and Resources [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income				441,528
First energy [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income				492,872
Duke Energy [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income	107,915		297,836
Total Reportable Segments [Member]
Segment Reporting Information [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	90,032	62,522	417,717	262,714
Total Assets	11,021,287	10,112,423	11,021,287	10,112,423
Gas
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	257,358	192,781	753,399	531,859
Revenue from Purchased Oil and Gas	1,205	1,607	6,183	4,372
Royalty Interest Gas Revenue	17,610	15,506	62,590	46,737
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	485	601	1,937	2,363
Revenue, Net		210,495	824,109	585,331
Total Revenue and Other Income	276,658
Income (Loss) from Continuing Operations before Income Taxes, Domestic	37,685	(1,867)	138,604	(7,117)
Total Assets	6,901,696	5,994,072	6,901,696	5,994,072
Depreciation, Depletion and Amortization	82,538	58,998	225,766	164,832
Segment Reporting - Capital Expenditures	281,641	273,474	852,097	669,067
Equity in Earnings of Affiliates	9,991	5,307	22,801	9,519
Equity Method Investments	92,188	183,895	92,188	183,895
Gas | Marcellus Shale [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	109,850	72,406	339,391	167,394
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		72,406	339,391	167,394
Total Revenue and Other Income	109,850
Income (Loss) from Continuing Operations before Income Taxes, Domestic	27,328	27,963	121,197	53,411
Gas | Coalbed Methane [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	82,913	83,269	259,665	254,708
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	485	601	1,937	2,363
Revenue, Net		83,870	261,602	257,071
Total Revenue and Other Income	83,398
Income (Loss) from Continuing Operations before Income Taxes, Domestic	19,790	20,908	71,358	64,213
Gas | Conventional Gas [Member]
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	25,377	32,957	84,278	99,138
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		32,957	84,278	99,138
Total Revenue and Other Income	25,377
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(9,341)	(2,123)	(16,194)	(11,752)
Gas | Other Gas
Segment Reporting Information [Line Items]
Natural Gas, NGLs and Oil Sales	39,218	4,149	70,065	10,619
Freight-Outside	0	0	0	0
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		21,262	138,838	61,728
Total Revenue and Other Income	58,033
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(92)	(48,615)	(37,757)	(112,989)
Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside	483,960	479,311	1,554,939	1,532,280
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	2,497	9,579	22,551	31,492
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		488,890	1,577,490	1,563,772
Total Revenue and Other Income	486,457
Income (Loss) from Continuing Operations before Income Taxes, Domestic	52,347	64,389	279,113	269,831
Total Assets	4,119,591	4,118,351	4,119,591	4,118,351
Depreciation, Depletion and Amortization	64,880	57,265	186,029	169,702
Segment Reporting - Capital Expenditures	72,496	32,497	320,196	336,845
Equity in Earnings of Affiliates	6,842	(1,769)	16,635	10,525
Equity Method Investments	25,844	20,131	25,844	20,131
Coal Segment [Member] | Steam [Member]
Segment Reporting Information [Line Items]
Sales-Outside	398,863	352,362	1,262,248	1,034,228
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		352,362	1,262,248	1,034,228
Total Revenue and Other Income	398,863
Income (Loss) from Continuing Operations before Income Taxes, Domestic	81,660	95,916	342,133	277,289
Coal Segment [Member] | Low Volatile Metallurgical [Member]
Segment Reporting Information [Line Items]
Sales-Outside	69,850	98,232	221,162	356,066
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		98,232	221,162	356,066
Total Revenue and Other Income	69,850
Income (Loss) from Continuing Operations before Income Taxes, Domestic	9,115	21,297	29,837	106,832
Coal Segment [Member] | High Volatile Metallurgical [member]
Segment Reporting Information [Line Items]
Sales-Outside	14,684	22,290	64,099	124,957
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		22,290	64,099	124,957
Total Revenue and Other Income	14,684
Income (Loss) from Continuing Operations before Income Taxes, Domestic	8,056	4,801	24,731	32,307
Coal Segment [Member] | Other Coal [Member]
Segment Reporting Information [Line Items]
Sales-Outside	563	6,427	7,430	17,029
Revenue from Purchased Oil and Gas	0	0	0	0
Royalty Interest Gas Revenue	0	0	0	0
Freight-Outside	2,497	9,579	22,551	31,492
Segment Reporting - Intersegment Transfers	0	0	0	0
Revenue, Net		16,006	29,981	48,521
Total Revenue and Other Income	3,060
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(46,484)	(57,625)	(117,588)	(146,597)
Other Segments [Member]
Segment Reporting Information [Line Items]
Sales-Outside	73,673	63,876	213,047	197,778
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	23,066	32,213	62,412	100,118
Revenue, Net		96,089	275,459	297,896
Total Revenue and Other Income	96,739
Income (Loss) from Continuing Operations before Income Taxes, Domestic	2,689	(2,385)	5,941	5,076
Total Assets	295,279	356,731	295,279	356,731
Depreciation, Depletion and Amortization	1,247	1,467	3,885	4,303
Segment Reporting - Capital Expenditures	1,175	7,704	2,314	15,215
Equity in Earnings of Affiliates	132	72	(959)	232
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	(23,551)	(32,814)	(64,349)	(102,481)
Revenue, Net		(32,814)	(64,349)	(102,481)
Total Revenue and Other Income	(23,551)
Income (Loss) from Continuing Operations before Income Taxes, Domestic	(95,754)	(63,448)	(320,232)	(237,590)
Total Assets	402,369	2,267,918	402,369	2,267,918
Depreciation, Depletion and Amortization	0		0	0
Segment Reporting - Capital Expenditures	0		0	0
All Other
Segment Reporting Information [Line Items]
Equity Method Investments	$ 67,477	$ 57,192	$ 67,477	$ 57,192