Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 117,770	$ 138,149	$ 69,220	$ 93,593
Restricted cash	8,866	10,322
Receivables	361,714	469,179
Inventories	488,686	377,490
Other	202,083	187,634
Total current assets	1,179,119	1,182,774
Property, plant and equipment, net	7,892,733	7,949,150
Other	1,093,266	1,082,035
Total other assets	1,093,266	1,082,035
Total assets	10,165,118	10,213,959
Liabilities and Stockholders' Equity
Accounts payable	294,341	383,782
Accrued expenses and other current liabilities	366,486	356,035
Current maturities of debt and short-term borrowings	102,356	280,851
Total current liabilities	763,183	1,020,668
Long-term debt	3,967,796	3,762,297
Asset retirement obligations	432,620	446,784
Accrued pension benefits	49,378	48,244
Accrued postretirement benefits other than pension	42,784	42,309
Accrued workers' compensation	74,012	71,948
Deferred income taxes	982,596	976,753
Other noncurrent liabilities	268,585	255,382
Total liabilities	6,580,954	6,624,385
Redeemable noncontrolling interest	11,739	11,534
Stockholders' equity	3,572,425	3,578,040
Total liabilities and stockholders' equity	10,165,118	10,213,959
Parent/Issuer [Member]
Assets
Cash and cash equivalents	35,196	61,375	3,056	13,713
Restricted cash	8,866	10,322
Receivables	46,832	65,187
Other	108,080	81,732
Total current assets	198,974	218,616
Property, plant and equipment, net	29,966	21,241
Investment in subsidiaries	8,886,286	8,805,731
Intercompany receivables	(1,443,557)	(1,457,864)
Note receivable from Arch Western	225,000	225,000
Other	450,008	184,266
Total other assets	8,117,737	7,757,133
Total assets	8,346,677	7,996,990
Liabilities and Stockholders' Equity
Accounts payable	26,036	25,409
Accrued expenses and other current liabilities	98,109	75,133
Current maturities of debt and short-term borrowings	10,876	172,564
Total current liabilities	135,021	273,106
Long-term debt	3,514,463	3,308,674
Asset retirement obligations	891	877
Accrued pension benefits	21,409	19,198
Accrued postretirement benefits other than pension	13,731	13,843
Accrued workers' compensation	21,350	17,272
Deferred income taxes	905,635	621,483
Other noncurrent liabilities	150,013
Total liabilities	4,762,513	4,407,416
Redeemable noncontrolling interest	11,739	11,534
Stockholders' equity	3,572,425	3,578,040
Total liabilities and stockholders' equity	8,346,677	7,996,990
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	308	332	49	64
Receivables	20,771	22,037
Inventories	256,896
Other	79,817	83,122
Total current assets	357,792	312,541
Property, plant and equipment, net	6,367,590	6,403,658
Intercompany receivables	(41,436)	7,010
Other	626,788	884,613
Total other assets	585,352	891,623
Total assets	7,310,734	7,607,822
Liabilities and Stockholders' Equity
Accounts payable	134,697	175,196
Accrued expenses and other current liabilities	110,821	115,685
Current maturities of debt and short-term borrowings	1,180	1,987
Total current liabilities	246,698	292,868
Long-term debt	2,524	2,652
Asset retirement obligations	142,014	140,861
Accrued pension benefits	4,001	4,203
Accrued postretirement benefits other than pension	6,272	6,271
Accrued workers' compensation	45,823	48,111
Deferred income taxes	76,961	355,270
Other noncurrent liabilities	64,098
Total liabilities	588,391	915,031
Stockholders' equity	6,722,343	6,692,791
Total liabilities and stockholders' equity	7,310,734	7,607,822
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	82,266	76,442	66,115	79,816
Receivables	295,844	383,572
Inventories	231,790
Other	14,186	22,780
Total current assets	624,086	653,234
Property, plant and equipment, net	1,495,177	1,524,251
Intercompany receivables	1,484,993	1,450,854
Other	16,470	13,156
Total other assets	1,501,463	1,464,010
Total assets	3,620,726	3,641,495
Liabilities and Stockholders' Equity
Accounts payable	133,608	183,177
Accrued expenses and other current liabilities	159,289	166,834
Current maturities of debt and short-term borrowings	90,300	106,300
Total current liabilities	383,197	456,311
Long-term debt	450,809	450,971
Note payable to Arch Coal	225,000	225,000
Asset retirement obligations	289,715	305,046
Accrued pension benefits	23,968	24,843
Accrued postretirement benefits other than pension	22,781	22,195
Accrued workers' compensation	6,839	6,565
Other noncurrent liabilities	54,474
Total liabilities	1,456,783	1,528,555
Stockholders' equity	2,163,943	2,112,940
Total liabilities and stockholders' equity	3,620,726	3,641,495
Eliminations [Member]
Assets
Receivables	(1,733)	(1,617)
Total current assets	(1,733)	(1,617)
Investment in subsidiaries	(8,886,286)	(8,805,731)
Note receivable from Arch Western	(225,000)	(225,000)
Total other assets	(9,111,286)	(9,030,731)
Total assets	(9,113,019)	(9,032,348)
Liabilities and Stockholders' Equity
Accrued expenses and other current liabilities	(1,733)	(1,617)
Total current liabilities	(1,733)	(1,617)
Note payable to Arch Coal	(225,000)	(225,000)
Total liabilities	(226,733)	(226,617)
Stockholders' equity	(8,886,286)	(8,805,731)
Total liabilities and stockholders' equity	$ (9,113,019)	$ (9,032,348)