Segment Information Industry Segment Results (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Entity-Wide Revenue, Major Customer, Amount									$ 662,109
Sales-Outside									5,660,813	4,938,703	4,311,791
Sales-Purchased Gas									4,344	11,227	7,040
Sales-Gas Royalty Interests									66,929	62,869	40,951
Freight-Outside	75,225	59,871	59,572	36,868	29,171	37,269	28,075	31,200	231,536	125,715	148,907
Intersegment Transfers									0	0	0
Total Sales and Freight									5,963,622	5,138,514	4,508,689
Earnings Before Income Taxes									787,953	467,913	788,345
Total Assets	12,525,700				12,070,610				12,525,700	12,070,610	7,775,401
Depreciation, Depletion and Amortization									618,397	567,663	437,417
Capital Expenditures									1,382,371	4,624,236	920,080
Equity in Earnings of Affiliates									(24,663)	(21,428)	(15,707)
Investments in Unconsolidated Equity Affiliates	182,036				93,509				182,036	93,509
Payments to Acquire Businesses, Net of Cash Acquired									0	3,470,212	0
Total Reportable Business Segments
Segment Reporting Information [Line Items]
Earnings Before Income Taxes									1,062,933	716,372	810,177
Total Assets	11,436,808				10,972,676				11,436,808	10,972,676	6,894,003
Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									4,566,848	3,899,389	3,409,886
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									231,536	125,715	148,907
Intersegment Transfers									0	0	0
Total Sales and Freight									4,798,384	4,025,104	3,558,793
Earnings Before Income Taxes									933,149	536,494	546,729
Total Assets	5,253,226				5,056,583				5,253,226	5,056,583	4,722,508
Depreciation, Depletion and Amortization									392,765	359,497	310,346
Capital Expenditures									676,587	707,473	580,401
Equity in Earnings of Affiliates									(15,803)	(13,846)	(5,663)
Investments in Unconsolidated Equity Affiliates	34,316				21,463				34,316	21,463	12,569
Gas
Segment Reporting Information [Line Items]
Sales-Outside									748,464	742,556	628,929
Sales-Purchased Gas									4,344	11,227	7,040
Sales-Gas Royalty Interests									66,929	62,869	40,951
Freight-Outside									0	0	0
Intersegment Transfers									3,303	3,253	1,671
Total Sales and Freight									823,040	819,905	678,591
Earnings Before Income Taxes									129,784	179,878	263,448
Total Assets	6,183,582				5,916,093				6,183,582	5,916,093	2,171,495
Depreciation, Depletion and Amortization									206,821	190,424	107,251
Capital Expenditures									664,612	3,891,640	322,126
Equity in Earnings of Affiliates									(4,231)	(479)	(636)
Investments in Unconsolidated Equity Affiliates	96,914				23,569				96,914	23,569	24,590
All Other Segments [Member]
Segment Reporting Information [Line Items]
Sales-Outside									345,501	296,758	272,976
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									194,857	175,906	152,375
Total Sales and Freight									540,358	472,664	425,351
Earnings Before Income Taxes									17,983	22,156	15,686
Total Assets	351,370				337,855				351,370	337,855	317,004
Depreciation, Depletion and Amortization									18,811	17,742	19,820
Capital Expenditures									41,172	25,123	17,553
Equity in Earnings of Affiliates									(4,629)	(7,103)
Corporate Elimination [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
Intersegment Transfers									(198,160)	(179,159)	(154,046)
Total Sales and Freight									(198,160)	(179,159)	(154,046)
Earnings Before Income Taxes									(292,963)	(270,615)	(37,518)
Total Assets	737,522				760,079				737,522	760,079	564,394
Depreciation, Depletion and Amortization									0	0	0
Capital Expenditures									0	0	0
All Other [Member]
Segment Reporting Information [Line Items]
Equity in Earnings of Affiliates											(9,408)
Investments in Unconsolidated Equity Affiliates	50,806				48,477				50,806	48,477	46,374
Steam [Member] | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									3,058,193	3,001,352	3,122,223
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									3,058,193	3,001,352	3,122,223
Earnings Before Income Taxes									456,306	460,697	718,947
Low Volatile Metallurgical | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									1,071,570	680,212	248,546
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									1,071,570	680,212	248,546
Earnings Before Income Taxes									680,495	381,562	93,688
High Volatile Metallurgical | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									368,221	172,087	0
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									368,221	172,087	0
Earnings Before Income Taxes									135,343	86,918	0
Other Coal | Coal Segment [Member]
Segment Reporting Information [Line Items]
Sales-Outside									68,864	45,738	39,117
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									231,536	125,715	148,907
Intersegment Transfers									0	0	0
Total Sales and Freight									300,400	171,453	188,024
Earnings Before Income Taxes									(338,995)	(392,683)	(265,906)
Coalbed Methane | Gas
Segment Reporting Information [Line Items]
Sales-Outside									462,677	569,367	595,769
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									462,677	569,367	595,769
Earnings Before Income Taxes									154,486	248,127	303,882
Marcellus Shale | Gas
Segment Reporting Information [Line Items]
Sales-Outside									118,973	48,769	21,006
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									118,973	48,769	21,006
Earnings Before Income Taxes									35,641	5,910	3,940
Conventional Gas | Gas
Segment Reporting Information [Line Items]
Sales-Outside									155,444	116,679	7,907
Sales-Purchased Gas									0	0	0
Sales-Gas Royalty Interests									0	0	0
Freight-Outside									0	0	0
Intersegment Transfers									0	0	0
Total Sales and Freight									155,444	116,679	7,907
Earnings Before Income Taxes									(23,151)	(4,179)	(2,259)
Other Gas | Gas
Segment Reporting Information [Line Items]
Sales-Outside									11,370	7,741	4,247
Sales-Purchased Gas									4,344	11,227	7,040
Sales-Gas Royalty Interests									66,929	62,869	40,951
Freight-Outside									0	0	0
Intersegment Transfers									3,303	3,253	1,671
Total Sales and Freight									85,946	85,090	53,909
Earnings Before Income Taxes									$ (37,192)	$ (69,980)	$ (42,115)