Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets
Cash and cash equivalents	$ 784,622	$ 138,149	$ 93,593	$ 61,138
Restricted cash	3,453	10,322
Short term investments	234,305
Receivables	332,080	469,179
Inventories	365,424	377,490
Other	194,220	187,634
Total current assets	1,914,104	1,182,774
Property, plant and equipment, net	7,337,098	7,949,150
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	755,575	1,082,035
Assets, Noncurrent	755,575	1,082,035
Total assets	10,006,777	10,213,959	4,880,769
Liabilities and Stockholders’ Equity
Accounts payable	224,418	383,782
Accrued expenses and other current liabilities	319,755	356,035
Current maturities of debt and short-term borrowings	32,896	280,851
Total current liabilities	577,069	1,020,668
Long-term debt	5,085,879	3,762,297
Note payable to Arch Coal
Asset retirement obligations	409,705	446,784
Accrued pension benefits	67,630	48,244
Accrued postretirement benefits other than pension	45,086	42,309
Accrued workers' compensation	81,629	71,948
Deferred income taxes	664,182	976,753
Other noncurrent liabilities	221,030	255,382
Total liabilities	7,152,210	6,624,385
Redeemable noncontrolling interest		11,534
Stockholders' equity	2,854,567	3,578,040	2,237,507	2,115,106
Total liabilities and stockholders' equity	10,006,777	10,213,959
Parent/Issuer [Member]
Assets
Cash and cash equivalents	671,313	61,375	13,713	54,255
Restricted cash	3,453	10,322
Short term investments	234,305
Receivables	49,281	65,187
Inventories
Other	106,786	81,732
Total current assets	1,065,138	218,616
Property, plant and equipment, net	27,476	21,241
Investment in subsidiaries	8,254,508	8,813,080
Intercompany receivables	(1,367,739)	(1,190,342)
Note receivable from Arch Western	675,000	225,000
Other	187,171	(85,668)
Assets, Noncurrent	7,748,940	7,762,070
Total assets	8,841,554	8,001,927
Liabilities and Stockholders’ Equity
Accounts payable	19,859	30,576
Accrued expenses and other current liabilities	65,293	75,121
Current maturities of debt and short-term borrowings	32,054	172,564
Total current liabilities	117,206	278,261
Long-term debt	5,061,925	3,308,674
Note payable to Arch Coal
Asset retirement obligations	1,646	877
Accrued pension benefits	33,456	19,198
Accrued postretirement benefits other than pension	13,953	13,843
Accrued workers' compensation	25,323	17,272
Deferred income taxes	664,182	621,483
Other noncurrent liabilities	69,296	152,745
Total liabilities	5,986,987	4,412,353
Redeemable noncontrolling interest		11,534
Stockholders' equity	2,854,567	3,578,040
Total liabilities and stockholders' equity	8,841,554	8,001,927
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,468	75,425	79,880	6,883
Restricted cash
Short term investments
Receivables	40,452	27,001
Inventories	365,424	377,490
Other	86,877	105,282
Total current assets	593,221	585,198
Property, plant and equipment, net	7,309,550	7,918,816
Investment in subsidiaries
Intercompany receivables	1,600,311	1,448,902
Note receivable from Arch Western
Other	568,314	1,167,501
Assets, Noncurrent	2,168,625	2,616,403
Total assets	10,071,396	11,120,417
Liabilities and Stockholders’ Equity
Accounts payable	204,370	353,180
Accrued expenses and other current liabilities	259,162	282,446
Current maturities of debt and short-term borrowings	842	1,987
Total current liabilities	464,374	637,613
Long-term debt	23,954	453,623
Note payable to Arch Coal	675,000	225,000
Asset retirement obligations	408,059	445,907
Accrued pension benefits	34,174	29,046
Accrued postretirement benefits other than pension	31,133	28,466
Accrued workers' compensation	56,306	54,676
Deferred income taxes		355,270
Other noncurrent liabilities	151,360	102,553
Total liabilities	1,844,360	2,332,154
Redeemable noncontrolling interest
Stockholders' equity	8,227,036	8,788,263
Total liabilities and stockholders' equity	10,071,396	11,120,417
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	12,841	1,349
Restricted cash
Short term investments
Receivables	247,171	378,608
Inventories
Other	557	620
Total current assets	260,569	380,577
Property, plant and equipment, net	72	9,093
Investment in subsidiaries
Intercompany receivables	(232,572)	(258,560)
Note receivable from Arch Western
Other	90	202
Assets, Noncurrent	(232,482)	(258,358)
Total assets	28,159	131,312
Liabilities and Stockholders’ Equity
Accounts payable	189	26
Accrued expenses and other current liabilities	124	85
Current maturities of debt and short-term borrowings		106,300
Total current liabilities	313	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	374	84
Total liabilities	687	106,495
Redeemable noncontrolling interest
Stockholders' equity	27,472	24,817
Total liabilities and stockholders' equity	28,159	131,312
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,824)	(1,617)
Inventories
Other
Total current assets	(4,824)	(1,617)
Property, plant and equipment, net
Investment in subsidiaries	(8,254,508)	(8,813,080)
Intercompany receivables
Note receivable from Arch Western	(675,000)	(225,000)
Other
Assets, Noncurrent	(8,929,508)	(9,038,080)
Total assets	(8,934,332)	(9,039,697)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,824)	(1,617)
Current maturities of debt and short-term borrowings
Total current liabilities	(4,824)	(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	(679,824)	(226,617)
Redeemable noncontrolling interest
Stockholders' equity	(8,254,508)	(8,813,080)
Total liabilities and stockholders' equity	$ (8,934,332)	$ (9,039,697)