Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 1,133,128	$ 784,622	$ 550,756	$ 138,149
Restricted cash		3,453
Short term investments	248,724	234,305
Receivables	214,839	332,080
Inventories	287,409	365,424
Other	145,539	194,220
Total current assets	2,029,639	1,914,104
Property, plant and equipment, net	6,778,225	7,337,098
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	723,591	755,575
Assets, Noncurrent	723,591	755,575
Total assets	9,531,455	10,006,777	9,935,854
Liabilities and Stockholders’ Equity
Accounts payable	207,273	224,418
Accrued expenses and other current liabilities	349,454	319,755
Current maturities of debt	23,050	32,896
Total current liabilities	579,777	577,069
Long-term debt	5,074,384	5,085,879
Intercompany payables
Note payable to Arch Coal
Asset retirement obligations	411,121	409,705
Accrued pension benefits	68,539	67,630
Accrued postretirement benefits other than pension	44,279	45,086
Accrued workers’ compensation	82,014	81,629
Deferred income taxes	484,130	664,182
Other noncurrent liabilities	205,557	221,030
Total liabilities	6,949,801	7,152,210
Stockholders' equity	2,581,654	2,854,567
Total liabilities and stockholders’ equity	9,531,455	10,006,777
Parent/Issuer [Member]
Assets
Cash and cash equivalents	1,021,245	671,313	313,548	66,542
Restricted cash		3,453
Short term investments	248,724	234,305
Receivables	7,847	49,281
Inventories
Other	96,992	106,786
Total current assets	1,374,808	1,065,138
Property, plant and equipment, net	25,526	27,476
Investment in subsidiaries	7,992,601	8,254,508
Intercompany receivables
Note receivable from Arch Western	675,000	675,000
Other	147,950	187,171
Assets, Noncurrent	8,815,551	9,116,679
Total assets	10,215,885	10,209,293
Liabilities and Stockholders’ Equity
Accounts payable	20,728	19,859
Accrued expenses and other current liabilities	107,756	65,293
Current maturities of debt	19,343	32,054
Total current liabilities	147,827	117,206
Long-term debt	5,055,021	5,061,925
Intercompany payables	1,810,718	1,367,739
Note payable to Arch Coal
Asset retirement obligations	1,150	1,646
Accrued pension benefits	36,304	33,456
Accrued postretirement benefits other than pension	14,267	13,953
Accrued workers’ compensation	25,601	25,323
Deferred income taxes	484,130	664,182
Other noncurrent liabilities	59,179	69,296
Total liabilities	7,634,197	7,354,726
Stockholders' equity	2,581,688	2,854,567
Total liabilities and stockholders’ equity	10,215,885	10,209,293
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,410	100,468	225,431	70,258
Restricted cash
Short term investments
Receivables	21,178	40,452
Inventories	287,409	365,424
Other	48,270	86,877
Total current assets	457,267	593,221
Property, plant and equipment, net	6,752,687	7,309,550
Investment in subsidiaries
Intercompany receivables	1,986,992	1,600,311
Note receivable from Arch Western
Other	575,551	568,314
Assets, Noncurrent	2,562,543	2,168,625
Total assets	9,772,497	10,071,396
Liabilities and Stockholders’ Equity
Accounts payable	186,468	204,370
Accrued expenses and other current liabilities	246,063	259,162
Current maturities of debt	3,707	842
Total current liabilities	436,238	464,374
Long-term debt	19,363	23,954
Intercompany payables
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	409,971	408,059
Accrued pension benefits	32,235	34,174
Accrued postretirement benefits other than pension	30,012	31,133
Accrued workers’ compensation	56,413	56,306
Deferred income taxes
Other noncurrent liabilities	146,055	151,360
Total liabilities	1,805,287	1,844,360
Stockholders' equity	7,967,210	8,227,036
Total liabilities and stockholders’ equity	9,772,497	10,071,396
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,473	12,841	11,777	1,349
Restricted cash
Short term investments
Receivables	190,493	247,171
Inventories
Other	277	557
Total current assets	202,243	260,569
Property, plant and equipment, net	46	72
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	90	90
Assets, Noncurrent	90	90
Total assets	202,379	260,731
Liabilities and Stockholders’ Equity
Accounts payable	77	189
Accrued expenses and other current liabilities	314	124
Current maturities of debt
Total current liabilities	391	313
Long-term debt
Intercompany payables	176,274	232,572
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	323	374
Total liabilities	176,988	233,259
Stockholders' equity	25,391	27,472
Total liabilities and stockholders’ equity	202,379	260,731
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,679)	(4,824)
Inventories
Other
Total current assets	(4,679)	(4,824)
Property, plant and equipment, net	(34)
Investment in subsidiaries	(7,992,601)	(8,254,508)
Intercompany receivables	(1,986,992)	(1,600,311)
Note receivable from Arch Western	(675,000)	(675,000)
Other
Assets, Noncurrent	(10,654,593)	(10,529,819)
Total assets	(10,659,306)	(10,534,643)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,679)	(4,824)
Current maturities of debt
Total current liabilities	(4,679)	(4,824)
Long-term debt
Intercompany payables	(1,986,992)	(1,600,311)
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	(2,666,671)	(2,280,135)
Stockholders' equity	(7,992,635)	(8,254,508)
Total liabilities and stockholders’ equity	$ (10,659,306)	$ (10,534,643)