Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Assets
Cash and cash equivalents	$ 138,149	$ 93,593	$ 61,138	$ 70,649
Restricted cash	10,322
Receivables	469,179	252,320
Inventories	377,490	235,616
Other	187,634	153,385
Total current assets	1,182,774	734,914
Property, plant and equipment, net	7,949,150	3,308,892
Other	1,082,035	836,963
Total other assets	1,082,035	836,963
Total assets	10,213,959	4,880,769	4,840,596
Liabilities and Stockholders' Equity
Accounts payable	383,782	198,216
Accrued expenses and other current liabilities	356,035	258,133
Current maturities of debt and short-term borrowings	280,851	70,997
Total current liabilities	1,020,668	527,346
Long-term debt	3,762,297	1,538,744
Asset retirement obligations	446,784	334,257
Accrued pension benefits	48,244	49,154
Accrued postretirement benefits other than pension	42,309	37,793
Accrued workers' compensation	71,948	35,290
Deferred income taxes	976,753
Other noncurrent liabilities	255,382	110,234
Total liabilities	6,624,385	2,632,818
Redeemable noncontrolling interest	11,534	10,444
Stockholders' equity	3,578,040	2,237,507	2,115,106	1,728,733
Total liabilities and stockholders' equity	10,213,959	4,880,769
Parent/Issuer [Member]
Assets
Cash and cash equivalents	61,375	13,713	54,255	67,737
Restricted cash	10,322
Receivables	65,187	31,458
Other	81,732	29,575
Total current assets	218,616	74,746
Property, plant and equipment, net	21,241	9,817
Investment in subsidiaries	8,805,731	4,555,233
Intercompany receivables	(1,457,864)	(1,807,902)
Note receivable from Arch Western	225,000	225,000
Other	184,266	481,345
Total other assets	7,757,133	3,453,676
Total assets	7,996,990	3,538,239
Liabilities and Stockholders' Equity
Accounts payable	25,409	10,753
Accrued expenses and other current liabilities	75,133	75,746
Current maturities of debt and short-term borrowings	172,564	14,093
Total current liabilities	273,106	100,592
Long-term debt	3,308,674	1,087,126
Asset retirement obligations	877	873
Accrued pension benefits	19,198	20,843
Accrued postretirement benefits other than pension	13,843	14,284
Accrued workers' compensation	17,272	15,383
Deferred income taxes	621,483
Other noncurrent liabilities	152,963	51,187
Total liabilities	4,407,416	1,290,288
Redeemable noncontrolling interest	11,534	10,444
Stockholders' equity	3,578,040	2,237,507
Total liabilities and stockholders' equity	7,996,990	3,538,239
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	332	64	64	61
Receivables	22,037	12,740
Inventories	207,050	85,196
Other	83,122	102,375
Total current assets	312,541	200,375
Property, plant and equipment, net	6,403,658	1,800,578
Intercompany receivables	7,010	508,624
Other	884,613	344,698
Total other assets	891,623	853,322
Total assets	7,607,822	2,854,275
Liabilities and Stockholders' Equity
Accounts payable	175,196	65,793
Accrued expenses and other current liabilities	115,685	31,123
Current maturities of debt and short-term borrowings	1,987
Total current liabilities	292,868	96,916
Long-term debt	2,652
Asset retirement obligations	140,861	32,029
Accrued pension benefits	4,203	4,407
Accrued postretirement benefits other than pension	6,271
Accrued workers' compensation	48,111	13,805
Deferred income taxes	355,270
Other noncurrent liabilities	64,795	22,135
Total liabilities	915,031	169,292
Stockholders' equity	6,692,791	2,684,983
Total liabilities and stockholders' equity	7,607,822	2,854,275
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	76,442	79,816	6,819	2,851
Receivables	383,572	210,075
Inventories	170,440	150,420
Other	22,780	21,435
Total current assets	653,234	461,746
Property, plant and equipment, net	1,524,251	1,498,497
Intercompany receivables	1,450,854	1,299,278
Other	13,156	10,920
Total other assets	1,464,010	1,310,198
Total assets	3,641,495	3,270,441
Liabilities and Stockholders' Equity
Accounts payable	183,177	121,670
Accrued expenses and other current liabilities	166,834	153,217
Current maturities of debt and short-term borrowings	106,300	56,904
Total current liabilities	456,311	331,791
Long-term debt	450,971	451,618
Note payable to Arch Coal	225,000	225,000
Asset retirement obligations	305,046	301,355
Accrued pension benefits	24,843	23,904
Accrued postretirement benefits other than pension	22,195	23,509
Accrued workers' compensation	6,565	6,102
Other noncurrent liabilities	37,624	36,912
Total liabilities	1,528,555	1,400,191
Stockholders' equity	2,112,940	1,870,250
Total liabilities and stockholders' equity	3,641,495	3,270,441
Eliminations [Member]
Assets
Receivables	(1,617)	(1,953)
Total current assets	(1,617)	(1,953)
Investment in subsidiaries	(8,805,731)	(4,555,233)
Note receivable from Arch Western	(225,000)	(225,000)
Total other assets	(9,030,731)	(4,780,233)
Total assets	(9,032,348)	(4,782,186)
Liabilities and Stockholders' Equity
Accrued expenses and other current liabilities	(1,617)	(1,953)
Total current liabilities	(1,617)	(1,953)
Note payable to Arch Coal	(225,000)	(225,000)
Total liabilities	(226,617)	(226,953)
Stockholders' equity	(8,805,731)	(4,555,233)
Total liabilities and stockholders' equity	$ (9,032,348)	$ (4,782,186)