Segment Information Industry Segment Results (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Sales-Outside									$ 3,015,551	$ 3,122,550	$ 3,991,007
Sales-Purchased Gas									6,531	3,316	4,344
Sales-Gas Royalty Interests									63,202	49,405	66,929
Freight-Outside	3,946	11,563	13,426	10,125	14,061	27,430	49,472	49,293	35,438	107,079	175,633
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									3,299,685	3,677,526	4,377,045
Revenue From External Customers									3,120,722	3,282,350	4,237,913
Earnings Before Income Taxes									46,075	406,687	872,926
Total Assets	11,393,667		12,997,594						11,393,667	12,997,594
Depreciation, Depletion and Amortization									461,122	427,115	430,577
Segment Reporting - Capital Expenditures									1,496,056	1,245,497	1,178,375
Equity in Earnings of Affiliates									(33,133)	(27,048)	(24,663)
Investments in Unconsolidated Equity Affiliates	291,675		222,830						291,675	222,830
Total Reportable Business Segments
Segment Reporting Information [Line Items]
Earnings Before Income Taxes									334,910	631,078	1,162,481
Total Assets	10,521,753		9,873,863						10,521,753	9,873,863
Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									2,018,067	2,169,625	2,957,760
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									35,438	107,079	175,633
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									2,053,505	2,276,704	3,133,393
Earnings Before Income Taxes									336,524	591,627	1,032,697
Total Assets	4,187,285		4,104,981						4,187,285	4,104,981
Depreciation, Depletion and Amortization									218,414	211,831	214,285
Segment Reporting - Capital Expenditures									458,653	662,888	472,591
Equity in Earnings of Affiliates									(17,346)	(17,318)	(19,629)
Investments in Unconsolidated Equity Affiliates	20,512		19,517						20,512	19,517
Gas
Segment Reporting Information [Line Items]
Sales-Outside									737,701	658,820	748,464
Sales-Purchased Gas									6,531	3,316	4,344
Sales-Gas Royalty Interests									63,202	49,405	66,929
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									(3,167)	1,622	(3,303)
Total Revenue and Other Income									810,601	713,163	823,040
Earnings Before Income Taxes									(1,614)	39,451	129,784
Total Assets	6,334,468		5,768,882						6,334,468	5,768,882
Depreciation, Depletion and Amortization									229,562	202,956	206,821
Segment Reporting - Capital Expenditures									968,607	532,636	664,612
Equity in Earnings of Affiliates									(14,684)	(9,562)	(4,231)
Investments in Unconsolidated Equity Affiliates	206,060		143,876						206,060	143,876
Other Segments [Member]
Segment Reporting Information [Line Items]
Sales-Outside									259,783	294,105	284,783
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									(127,553)	(142,014)	(194,857)
Total Revenue and Other Income									387,336	436,119	479,640
Earnings Before Income Taxes									(47,468)	10,997	3,408
Total Assets	293,486		363,676						293,486	363,676
Depreciation, Depletion and Amortization									13,146	12,328	9,471
Segment Reporting - Capital Expenditures									68,796	49,973	41,172
Equity in Earnings of Affiliates									(1,102)	(168)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Sales-Outside									0	0	0
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									130,720	143,636	198,160
Total Revenue and Other Income									(130,720)	(143,636)	(198,160)
Earnings Before Income Taxes									(241,367)	(235,388)	(292,963)
Total Assets	578,428		2,760,055						578,428	2,760,055
Depreciation, Depletion and Amortization									0	0	0
Segment Reporting - Capital Expenditures									0	0	0
All Other [Member]
Segment Reporting Information [Line Items]
Equity in Earnings of Affiliates											(803)
Investments in Unconsolidated Equity Affiliates	65,103		59,437						65,103	59,437
Steam [Member] | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									1,388,005	1,430,912	1,495,480
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									1,388,005	1,430,912	1,495,480
Earnings Before Income Taxes									375,787	396,748	421,683
Low Volatile Metallurgical | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									447,417	505,670	1,071,570
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									447,417	505,670	1,071,570
Earnings Before Income Taxes									121,285	210,133	692,249
High Volatile Metallurgical | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									159,888	210,153	324,377
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									159,888	210,153	324,377
Earnings Before Income Taxes									40,500	57,163	129,119
Other Coal | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									22,757	22,890	66,333
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									35,438	107,079	175,633
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									58,195	129,969	241,966
Earnings Before Income Taxes									(201,048)	(72,417)	(210,354)
Coalbed Methane | Gas
Segment Reporting Information [Line Items]
Sales-Outside									335,730	379,595	462,677
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									335,730	379,595	462,677
Earnings Before Income Taxes									81,392	125,970	185,761
Marcellus Shale | Gas
Segment Reporting Information [Line Items]
Sales-Outside									251,846	134,080	118,973
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									251,846	134,080	118,973
Earnings Before Income Taxes									79,439	29,546	41,566
Conventional Gas | Gas
Segment Reporting Information [Line Items]
Sales-Outside									131,135	135,412	155,444
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									0	0	0
Total Revenue and Other Income									131,135	135,412	155,444
Earnings Before Income Taxes									(17,829)	(13,390)	(14,732)
Other Gas | Gas
Segment Reporting Information [Line Items]
Sales-Outside									18,990	9,733	11,370
Sales-Purchased Gas									6,531	3,316	4,344
Sales-Gas Royalty Interests									63,202	49,405	66,929
Freight-Outside									0	0	0
Segment Reporting - Intersegment Transfers									(3,167)	(1,622)	(3,303)
Total Revenue and Other Income									91,890	64,076	85,946
Earnings Before Income Taxes									(144,616)	(102,675)	(82,811)
XCoal Energy and Resources [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income									495,242	382,843	655,596
Duke Energy [Member]
Segment Reporting Information [Line Items]
Total Revenue and Other Income									$ 346,424