Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 512,527	$ 138,149	$ 82,345	$ 93,593
Restricted cash	5,740	10,322
Receivables	397,505	469,179
Inventories	455,091	377,490
Other	197,664	187,634
Total current assets	1,568,527	1,182,774
Property, plant and equipment, net	7,397,131	7,949,150
Other	988,280	1,082,035
Total other assets	988,280	1,082,035
Total assets	9,953,938	10,213,959	10,256,548
Liabilities and Stockholders' Equity
Accounts payable	316,669	383,782
Accrued expenses and other current liabilities	367,883	356,035
Current maturities of debt and short-term borrowings	111,260	280,851
Total current liabilities	795,812	1,020,668
Long-term debt	4,464,351	3,762,297
Asset retirement obligations	424,289	446,784
Accrued pension benefits	49,040	48,244
Accrued postretirement benefits other than pension	42,028	42,309
Accrued workers' compensation	82,372	71,948
Deferred income taxes	730,495	976,753
Other noncurrent liabilities	223,131	255,382
Total liabilities	6,811,518	6,624,385
Redeemable noncontrolling interest	17,500	11,534
Stockholders' equity	3,124,920	3,578,040
Total liabilities and stockholders' equity	9,953,938	10,213,959
Parent/Issuer [Member]
Assets
Cash and cash equivalents	400,698	61,375	9,350	13,713
Restricted cash	5,740	10,322
Receivables	47,525	65,187
Other	96,561	81,732
Total current assets	550,524	218,616
Property, plant and equipment, net	29,658	21,241
Investment in subsidiaries	8,337,749	8,805,731
Intercompany receivables	(1,195,860)	(1,457,864)
Note receivable from Arch Western	675,000	225,000
Other	189,359	184,266
Total other assets	8,006,248	7,757,133
Total assets	8,586,430	7,996,990
Liabilities and Stockholders' Equity
Accounts payable	29,584	25,409
Accrued expenses and other current liabilities	64,302	75,133
Current maturities of debt and short-term borrowings	20,100	172,564
Total current liabilities	113,986	273,106
Long-term debt	4,462,255	3,308,674
Asset retirement obligations	776	877
Accrued pension benefits	22,737	19,198
Accrued postretirement benefits other than pension	12,697	13,843
Accrued workers' compensation	27,675	17,272
Deferred income taxes	653,534	621,483
Other noncurrent liabilities	150,350	152,963
Total liabilities	5,444,010	4,407,416
Redeemable noncontrolling interest	17,500	11,534
Stockholders' equity	3,124,920	3,578,040
Total liabilities and stockholders' equity	8,586,430	7,996,990
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	360	332	2,134	64
Receivables	23,006	22,037
Inventories	232,126	207,050
Other	88,695	83,122
Total current assets	344,187	312,541
Property, plant and equipment, net	5,891,377	6,403,658
Intercompany receivables	(212,975)	7,010
Other	783,012	884,613
Total other assets	570,037	891,623
Total assets	6,805,601	7,607,822
Liabilities and Stockholders' Equity
Accounts payable	157,281	175,196
Accrued expenses and other current liabilities	157,977	115,685
Current maturities of debt and short-term borrowings	860	1,987
Total current liabilities	316,118	292,868
Long-term debt	2,396	2,652
Asset retirement obligations	153,423	140,861
Accrued pension benefits	3,781	4,203
Accrued postretirement benefits other than pension	6,272	6,271
Accrued workers' compensation	47,780	48,111
Deferred income taxes	76,961	355,270
Other noncurrent liabilities	41,016	64,795
Total liabilities	647,747	915,031
Stockholders' equity	6,157,854	6,692,791
Total liabilities and stockholders' equity	6,805,601	7,607,822
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	111,469	76,442	70,861	79,816
Receivables	330,188	383,572
Inventories	222,965	170,440
Other	12,408	22,780
Total current assets	677,030	653,234
Property, plant and equipment, net	1,476,096	1,524,251
Intercompany receivables	1,408,835	1,450,854
Other	15,909	13,156
Total other assets	1,424,744	1,464,010
Total assets	3,577,870	3,641,495
Liabilities and Stockholders' Equity
Accounts payable	129,804	183,177
Accrued expenses and other current liabilities	148,818	166,834
Current maturities of debt and short-term borrowings	90,300	106,300
Total current liabilities	368,922	456,311
Long-term debt	(300)	450,971
Note payable to Arch Coal	675,000	225,000
Asset retirement obligations	270,090	305,046
Accrued pension benefits	22,522	24,843
Accrued postretirement benefits other than pension	23,059	22,195
Accrued workers' compensation	6,917	6,565
Other noncurrent liabilities	31,765	37,624
Total liabilities	1,397,975	1,528,555
Stockholders' equity	2,179,895	2,112,940
Total liabilities and stockholders' equity	3,577,870	3,641,495
Eliminations [Member]
Assets
Receivables	(3,214)	(1,617)
Total current assets	(3,214)	(1,617)
Investment in subsidiaries	(8,337,749)	(8,805,731)
Note receivable from Arch Western	(675,000)	(225,000)
Total other assets	(9,012,749)	(9,030,731)
Total assets	(9,015,963)	(9,032,348)
Liabilities and Stockholders' Equity
Accrued expenses and other current liabilities	(3,214)	(1,617)
Total current liabilities	(3,214)	(1,617)
Note payable to Arch Coal	(675,000)	(225,000)
Total liabilities	(678,214)	(226,617)
Stockholders' equity	(8,337,749)	(8,805,731)
Total liabilities and stockholders' equity	$ (9,015,963)	$ (9,032,348)