Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 740,154	$ 911,099	$ 644,205	$ 784,622
Short term investments	248,647	248,414
Receivables	239,151	229,573
Inventories	228,726	264,161
Other	125,110	128,824
Total current assets	1,581,788	1,782,071
Property, plant and equipment, net	6,603,458	6,734,286
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	470,862	473,836
Total other assets	470,862	473,836
Total assets	8,656,108	8,990,193
Liabilities and Stockholders’ Equity
Accounts payable	160,209	176,142
Accrued expenses and other current liabilities	294,317	278,587
Current maturities of debt	27,266	33,493
Total current liabilities	481,792	488,222
Long-term debt	5,116,353	5,118,002
Intercompany payables
Note payable to Arch Coal
Asset retirement obligations	395,813	402,713
Accrued pension benefits	13,925	7,111
Accrued postretirement benefits other than pension	38,034	39,255
Accrued workers’ compensation	74,083	78,062
Deferred income taxes	332,207	413,546
Other noncurrent liabilities	172,512	190,033
Total liabilities	6,624,719	6,736,944
Stockholders' equity	2,031,389	2,253,249
Total liabilities and stockholders’ equity	8,656,108	8,990,193
Parent/Issuer [Member]
Assets
Cash and cash equivalents	628,251	799,333	531,805	671,313
Short term investments	248,647	248,414
Receivables	26,284	14,177
Inventories
Other	85,461	84,401
Total current assets	988,643	1,146,325
Property, plant and equipment, net	16,663	24,851
Investment in subsidiaries	7,643,340	7,709,201
Intercompany receivables	0
Note receivable from Arch Western	675,000	675,000
Other	152,974	162,287
Total other assets	8,471,314	8,546,488
Total assets	9,476,620	9,717,664
Liabilities and Stockholders’ Equity
Accounts payable	13,286	17,781
Accrued expenses and other current liabilities	70,523	53,779
Current maturities of debt	22,011	28,882
Total current liabilities	105,820	100,442
Long-term debt	5,092,064	5,099,833
Intercompany payables	1,820,942	1,740,236
Note payable to Arch Coal
Asset retirement obligations	1,037	1,095
Accrued pension benefits	2,869	7,797
Accrued postretirement benefits other than pension	4,374	12,079
Accrued workers’ compensation	27,214	21,546
Deferred income taxes	332,207	413,546
Other noncurrent liabilities	58,704	67,841
Total liabilities	7,445,231	7,464,415
Stockholders' equity	2,031,389	2,253,249
Total liabilities and stockholders’ equity	9,476,620	9,717,664
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,360	100,418	100,447	100,468
Short term investments
Receivables	14,060	23,018
Inventories	228,726	264,161
Other	38,039	43,617
Total current assets	381,185	431,214
Property, plant and equipment, net	6,586,776	6,709,398
Investment in subsidiaries
Intercompany receivables	2,009,804	1,921,331
Note receivable from Arch Western
Other	317,772	311,463
Total other assets	2,327,576	2,232,794
Total assets	9,295,537	9,373,406
Liabilities and Stockholders’ Equity
Accounts payable	146,871	158,224
Accrued expenses and other current liabilities	227,552	228,664
Current maturities of debt	5,255	4,611
Total current liabilities	379,678	391,499
Long-term debt	24,289	18,169
Intercompany payables
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	394,776	401,618
Accrued pension benefits	11,056	(686)
Accrued postretirement benefits other than pension	33,660	27,176
Accrued workers’ compensation	46,869	56,516
Deferred income taxes
Other noncurrent liabilities	113,525	121,794
Total liabilities	1,678,853	1,691,086
Stockholders' equity	7,616,684	7,682,320
Total liabilities and stockholders’ equity	9,295,537	9,373,406
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,543	11,348	11,953	12,841
Short term investments
Receivables	203,416	197,015
Inventories
Other	1,610	806
Total current assets	216,569	209,169
Property, plant and equipment, net	19	37
Investment in subsidiaries
Intercompany receivables	0
Note receivable from Arch Western
Other	116	86
Total other assets	116	86
Total assets	216,704	209,292
Liabilities and Stockholders’ Equity
Accounts payable	52	137
Accrued expenses and other current liabilities	851	781
Current maturities of debt
Total current liabilities	903	918
Long-term debt
Intercompany payables	188,862	181,095
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	283	398
Total liabilities	190,048	182,411
Stockholders' equity	26,656	26,881
Total liabilities and stockholders’ equity	216,704	209,292
Eliminations [Member]
Assets
Cash and cash equivalents
Short term investments
Receivables	(4,609)	(4,637)
Inventories
Other
Total current assets	(4,609)	(4,637)
Property, plant and equipment, net
Investment in subsidiaries	(7,643,340)	(7,709,201)
Intercompany receivables	(2,009,804)	(1,921,331)
Note receivable from Arch Western	(675,000)	(675,000)
Other
Total other assets	(10,328,144)	(10,305,532)
Total assets	(10,332,753)	(10,310,169)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,609)	(4,637)
Current maturities of debt
Total current liabilities	(4,609)	(4,637)
Long-term debt
Intercompany payables	(2,009,804)	(1,921,331)
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	(2,689,413)	(2,600,968)
Stockholders' equity	(7,643,340)	(7,709,201)
Total liabilities and stockholders’ equity	$ (10,332,753)	$ (10,310,169)