Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 550,756	$ 138,149	$ 158,509	$ 93,593
Restricted cash	3,450	10,322
Short term investments	99,359
Receivables	348,748	469,179
Inventories	391,526	377,490
Other	185,847	187,634
Total current assets	1,579,686	1,182,774
Property, plant and equipment, net	7,372,063	7,949,150
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	984,105	1,082,035
Assets, Noncurrent	984,105	1,082,035
Total assets	9,935,854	10,213,959	9,961,657
Accounts payable	304,442	383,782
Accrued expenses and other current liabilities	358,946	356,035
Current maturities of debt and short-term borrowings	115,695	280,851
Total current liabilities	779,083	1,020,668
Long-term debt	4,465,445	3,762,297
Note payable to Arch Coal
Asset retirement obligations	426,044	446,784
Accrued pension benefits	45,160	48,244
Accrued postretirement benefits other than pension	41,061	42,309
Accrued workers' compensation	85,251	71,948
Deferred income taxes	746,079	976,753
Other noncurrent liabilities	182,464	255,382
Total liabilities	6,770,587	6,624,385
Redeemable noncontrolling interest		11,534
Stockholders' equity	3,165,267	3,578,040
Total liabilities and stockholders' equity	9,935,854	10,213,959
Parent/Issuer [Member]
Assets
Cash and cash equivalents	313,548	61,375	57,778	13,713
Restricted cash	3,450	10,322
Short term investments	99,359
Receivables	52,081	65,187
Inventories
Other	104,507	81,732
Total current assets	572,945	218,616
Property, plant and equipment, net	28,622	21,241
Investment in subsidiaries	8,503,465	8,805,731
Intercompany receivables	(1,399,215)	(1,457,864)
Note receivable from Arch Western	675,000	225,000
Other	184,982	184,266
Assets, Noncurrent	7,964,232	7,757,133
Total assets	8,565,799	7,996,990
Accounts payable	35,157	25,409
Accrued expenses and other current liabilities	92,337	75,133
Current maturities of debt and short-term borrowings	14,832	172,564
Total current liabilities	142,326	273,106
Long-term debt	4,463,386	3,308,674
Note payable to Arch Coal
Asset retirement obligations	790	877
Accrued pension benefits	21,125	19,198
Accrued postretirement benefits other than pension	11,733	13,843
Accrued workers' compensation	28,129	17,272
Deferred income taxes	665,414	621,483
Other noncurrent liabilities	67,629	152,963
Total liabilities	5,400,532	4,407,416
Redeemable noncontrolling interest		11,534
Stockholders' equity	3,165,267	3,578,040
Total liabilities and stockholders' equity	8,565,799	7,996,990
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	225,431	75,425	100,419	79,880
Restricted cash
Short term investments
Receivables	24,672	27,001
Inventories	391,526	377,490
Other	80,901	105,282
Total current assets	722,530	585,198
Property, plant and equipment, net	7,343,360	7,918,816
Investment in subsidiaries
Intercompany receivables	1,561,073	1,716,424
Note receivable from Arch Western
Other	799,033	897,567
Assets, Noncurrent	2,360,106	2,613,991
Total assets	10,425,996	11,118,005
Accounts payable	269,203	358,347
Accrued expenses and other current liabilities	269,834	282,434
Current maturities of debt and short-term borrowings	863	1,987
Total current liabilities	539,900	642,768
Long-term debt	2,059	453,623
Note payable to Arch Coal	675,000	225,000
Asset retirement obligations	425,254	445,907
Accrued pension benefits	24,035	29,046
Accrued postretirement benefits other than pension	29,328	28,466
Accrued workers' compensation	57,122	54,676
Deferred income taxes	80,665	355,270
Other noncurrent liabilities	114,557	102,335
Total liabilities	1,947,920	2,337,091
Redeemable noncontrolling interest
Stockholders' equity	8,478,076	8,780,914
Total liabilities and stockholders' equity	10,425,996	11,118,005
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,777	1,349	312
Restricted cash
Short term investments
Receivables	276,992	378,608
Inventories
Other	439	620
Total current assets	289,208	380,577
Property, plant and equipment, net	81	9,093
Investment in subsidiaries
Intercompany receivables	(161,858)	(258,560)
Note receivable from Arch Western
Other	90	202
Assets, Noncurrent	(161,768)	(258,358)
Total assets	127,521	131,312
Accounts payable	82	26
Accrued expenses and other current liabilities	1,772	85
Current maturities of debt and short-term borrowings	100,000	106,300
Total current liabilities	101,854	106,411
Long-term debt
Note payable to Arch Coal
Asset retirement obligations
Accrued pension benefits
Accrued postretirement benefits other than pension
Accrued workers' compensation
Deferred income taxes
Other noncurrent liabilities	278	84
Total liabilities	102,132	106,495
Redeemable noncontrolling interest
Stockholders' equity	25,389	24,817
Total liabilities and stockholders' equity	127,521	131,312
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,997)	(1,617)
Inventories
Other
Total current assets	(4,997)	(1,617)
Property, plant and equipment, net
Investment in subsidiaries	(8,503,465)	(8,805,731)
Intercompany receivables
Note receivable from Arch Western	(675,000)	(225,000)
Other
Assets, Noncurrent	(9,178,465)	(9,030,731)
Total assets	(9,183,462)	(9,032,348)
Accounts payable
Accrued expenses and other current liabilities	(4,997)	(1,617)
Current maturities of debt and short-term borrowings
Total current liabilities	(4,997)	(1,617)
Long-term debt
Note payable to Arch Coal	(675,000)	(225,000)
Asset retirement obligations
Accrued pension benefits
Accrued postretirement benefits other than pension
Accrued workers' compensation
Deferred income taxes
Other noncurrent liabilities
Total liabilities	(679,997)	(226,617)
Redeemable noncontrolling interest
Stockholders' equity	(8,503,465)	(8,805,731)
Total liabilities and stockholders' equity	$ (9,183,462)	$ (9,032,348)