Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 644,205	$ 784,622	$ 512,527	$ 138,149
Restricted cash	1,085	3,453
Short term investments	248,464	234,305
Receivables	336,538	332,080
Inventories	356,653	365,424
Other	168,392	194,220
Total current assets	1,755,337	1,914,104
Property, plant and equipment, net	7,231,221	7,337,098
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	744,171	755,575
Assets, Noncurrent	744,171	755,575
Total assets	9,730,729	10,006,777	9,953,938
Liabilities and Stockholders’ Equity
Accounts payable	232,433	224,418
Accrued expenses and other current liabilities	307,631	319,755
Current maturities of debt	23,842	32,896
Total current liabilities	563,906	577,069
Long-term debt	5,078,634	5,085,879
Note payable to Arch Coal
Asset retirement obligations	414,860	409,705
Accrued pension benefits	66,903	67,630
Accrued postretirement benefits other than pension	46,358	45,086
Accrued workers’ compensation	79,004	81,629
Deferred income taxes	565,047	664,182
Other noncurrent liabilities	205,103	221,030
Total liabilities	7,019,815	7,152,210
Stockholders' equity	2,710,914	2,854,567
Total liabilities and stockholders’ equity	9,730,729	10,006,777
Parent/Issuer [Member]
Assets
Cash and cash equivalents	531,805	671,313	400,698	66,542
Restricted cash	1,085	3,453
Short term investments	248,464	234,305
Receivables	51,720	49,281
Inventories
Other	97,171	106,786
Total current assets	930,245	1,065,138
Property, plant and equipment, net	26,118	27,476
Investment in subsidiaries	8,291,665	8,254,508
Intercompany receivables	(1,496,992)	(1,367,739)
Note receivable from Arch Western	675,000	675,000
Other	164,358	187,171
Assets, Noncurrent	7,634,031	7,748,940
Total assets	8,590,394	8,841,554
Liabilities and Stockholders’ Equity
Accounts payable	29,870	19,859
Accrued expenses and other current liabilities	61,806	65,293
Current maturities of debt	20,790	32,054
Total current liabilities	112,466	117,206
Long-term debt	5,057,310	5,061,925
Note payable to Arch Coal
Asset retirement obligations	1,685	1,646
Accrued pension benefits	33,759	33,456
Accrued postretirement benefits other than pension	14,372	13,953
Accrued workers’ compensation	23,601	25,323
Deferred income taxes	565,047	664,182
Other noncurrent liabilities	71,240	69,296
Total liabilities	5,879,480	5,986,987
Stockholders' equity	2,710,914	2,854,567
Total liabilities and stockholders’ equity	8,590,394	8,841,554
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,447	100,468	100,453	70,258
Restricted cash
Short term investments
Receivables	25,508	40,452
Inventories	356,653	365,424
Other	70,748	86,877
Total current assets	553,356	593,221
Property, plant and equipment, net	7,205,048	7,309,550
Investment in subsidiaries
Intercompany receivables	1,746,169	1,600,311
Note receivable from Arch Western
Other	579,723	568,314
Assets, Noncurrent	2,325,892	2,168,625
Total assets	10,084,296	10,071,396
Liabilities and Stockholders’ Equity
Accounts payable	202,495	204,370
Accrued expenses and other current liabilities	250,096	259,162
Current maturities of debt	3,052	842
Total current liabilities	455,643	464,374
Long-term debt	21,324	23,954
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	413,175	408,059
Accrued pension benefits	33,144	34,174
Accrued postretirement benefits other than pension	31,986	31,133
Accrued workers’ compensation	55,403	56,306
Deferred income taxes
Other noncurrent liabilities	133,612	151,360
Total liabilities	1,819,287	1,844,360
Stockholders' equity	8,265,009	8,227,036
Total liabilities and stockholders’ equity	10,084,296	10,071,396
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,953	12,841	11,376	1,349
Restricted cash
Short term investments
Receivables	264,005	247,171
Inventories
Other	473	557
Total current assets	276,431	260,569
Property, plant and equipment, net	55	72
Investment in subsidiaries
Intercompany receivables	(249,177)	(232,572)
Note receivable from Arch Western
Other	90	90
Assets, Noncurrent	(249,087)	(232,482)
Total assets	27,399	28,159
Liabilities and Stockholders’ Equity
Accounts payable	68	189
Accrued expenses and other current liabilities	424	124
Current maturities of debt
Total current liabilities	492	313
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	251	374
Total liabilities	743	687
Stockholders' equity	26,656	27,472
Total liabilities and stockholders’ equity	27,399	28,159
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,695)	(4,824)
Inventories
Other
Total current assets	(4,695)	(4,824)
Property, plant and equipment, net
Investment in subsidiaries	(8,291,665)	(8,254,508)
Intercompany receivables
Note receivable from Arch Western	(675,000)	(675,000)
Other
Assets, Noncurrent	(8,966,665)	(8,929,508)
Total assets	(8,971,360)	(8,934,332)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,695)	(4,824)
Current maturities of debt
Total current liabilities	(4,695)	(4,824)
Long-term debt
Note payable to Arch Coal	(675,000)	(675,000)
Asset retirement obligations
Accrued pension benefits
Accrued postretirement benefits other than pension
Accrued workers’ compensation
Deferred income taxes
Other noncurrent liabilities
Total liabilities	(679,695)	(679,824)
Stockholders' equity	(8,291,665)	(8,254,508)
Total liabilities and stockholders’ equity	$ (8,971,360)	$ (8,934,332)