Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 865,761	$ 911,099	$ 730,119	$ 784,622
Short term investments	248,572	248,414
Receivables	274,812	229,573
Inventories	224,806	264,161
Other	123,377	128,824
Total current assets	1,737,328	1,782,071
Property, plant and equipment, net	6,616,144	6,734,286
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	469,712	473,836
Total other assets	469,712	473,836
Total assets	8,823,184	8,990,193
Liabilities and Stockholders’ Equity
Accounts payable	160,361	176,142
Accrued expenses and other current liabilities	328,561	278,587
Current maturities of debt	29,950	33,493
Total current liabilities	518,872	488,222
Long-term debt	5,112,995	5,118,002
Intercompany payables
Note payable to Arch Coal
Asset retirement obligations	390,408	402,713
Accrued pension benefits	10,484	7,111
Accrued postretirement benefits other than pension	37,995	39,255
Accrued workers’ compensation	75,817	78,062
Deferred income taxes	368,057	413,546
Other noncurrent liabilities	181,866	190,033
Total liabilities	6,696,494	6,736,944
Stockholders' equity	2,126,690	2,253,249
Total liabilities and stockholders’ equity	8,823,184	8,990,193
Parent/Issuer [Member]
Assets
Cash and cash equivalents	753,902	799,333	597,308	671,313
Short term investments	248,572	248,414
Receivables	33,115	14,177
Inventories
Other	80,500	84,401
Total current assets	1,116,089	1,146,325
Property, plant and equipment, net	23,765	24,851
Investment in subsidiaries	7,724,424	7,741,589
Intercompany receivables	0
Note receivable from Arch Western	675,000	675,000
Other	157,663	162,287
Total other assets	8,557,087	8,578,876
Total assets	9,696,941	9,750,052
Liabilities and Stockholders’ Equity
Accounts payable	18,326	17,781
Accrued expenses and other current liabilities	105,706	53,779
Current maturities of debt	25,481	28,882
Total current liabilities	149,513	100,442
Long-term debt	5,095,777	5,099,833
Intercompany payables	1,865,706	1,772,624
Note payable to Arch Coal
Asset retirement obligations	1,018	1,095
Accrued pension benefits	2,873	7,797
Accrued postretirement benefits other than pension	4,213	12,079
Accrued workers’ compensation	27,606	21,546
Deferred income taxes	368,057	413,546
Other noncurrent liabilities	55,522	67,841
Total liabilities	7,570,285	7,496,803
Stockholders' equity	2,126,656	2,253,249
Total liabilities and stockholders’ equity	9,696,941	9,750,052
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,353	100,418	120,429	100,468
Short term investments
Receivables	16,594	23,018
Inventories	224,806	264,161
Other	42,347	43,617
Total current assets	384,100	431,214
Property, plant and equipment, net	6,592,317	6,709,398
Investment in subsidiaries
Intercompany receivables	2,079,874	1,953,719
Note receivable from Arch Western
Other	311,976	311,463
Total other assets	2,391,850	2,265,182
Total assets	9,368,267	9,405,794
Liabilities and Stockholders’ Equity
Accounts payable	141,921	158,224
Accrued expenses and other current liabilities	226,757	228,664
Current maturities of debt	4,469	4,611
Total current liabilities	373,147	391,499
Long-term debt	17,218	18,169
Intercompany payables
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	389,390	401,618
Accrued pension benefits	7,611	(686)
Accrued postretirement benefits other than pension	33,782	27,176
Accrued workers’ compensation	48,211	56,516
Deferred income taxes
Other noncurrent liabilities	126,039	121,794
Total liabilities	1,670,398	1,691,086
Stockholders' equity	7,697,869	7,714,708
Total liabilities and stockholders’ equity	9,368,267	9,405,794
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,506	11,348	12,382	12,841
Short term investments
Receivables	229,737	197,015
Inventories
Other	530	806
Total current assets	241,773	209,169
Property, plant and equipment, net	28	37
Investment in subsidiaries
Intercompany receivables	0	(181,095)
Note receivable from Arch Western
Other	73	86
Total other assets	73	(181,009)
Total assets	241,874	28,197
Liabilities and Stockholders’ Equity
Accounts payable	114	137
Accrued expenses and other current liabilities	732	781
Current maturities of debt
Total current liabilities	846	918
Long-term debt
Intercompany payables	214,168
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	305	398
Total liabilities	215,319	1,316
Stockholders' equity	26,555	26,881
Total liabilities and stockholders’ equity	241,874	28,197
Eliminations [Member]
Assets
Cash and cash equivalents
Short term investments
Receivables	(4,634)	(4,637)
Inventories
Other
Total current assets	(4,634)	(4,637)
Property, plant and equipment, net	34
Investment in subsidiaries	(7,724,424)	(7,741,589)
Intercompany receivables	(2,079,874)	(1,772,624)
Note receivable from Arch Western	(675,000)	(675,000)
Other
Total other assets	(10,479,298)	(10,189,213)
Total assets	(10,483,898)	(10,193,850)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,634)	(4,637)
Current maturities of debt
Total current liabilities	(4,634)	(4,637)
Long-term debt
Intercompany payables	(2,079,874)	(1,772,624)
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	(2,759,508)	(2,452,261)
Stockholders' equity	(7,724,390)	(7,741,589)
Total liabilities and stockholders’ equity	$ (10,483,898)	$ (10,193,850)