Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$ 689,972	$ 734,231	$ 865,761	$ 911,099
Short term investments	249,518	248,954
Receivables	213,749	232,017
Inventories	240,113	190,253
Other	137,637	137,296
Total current assets	1,530,989	1,542,751
Property, plant and equipment, net	6,371,335	6,453,458
Other	412,616	433,514
Total other assets	412,616	433,514
Total assets	8,314,940	8,429,723
Liabilities and Stockholders’ Equity
Accounts payable	167,480	180,113
Accrued expenses and other current liabilities	321,503	302,396
Current maturities of debt	34,368	36,885
Total current liabilities	523,351	519,394
Long-term debt	5,117,982	5,123,485
Asset retirement obligations	404,844	398,896
Accrued pension benefits	14,436	16,260
Accrued postretirement benefits other than pension	34,453	32,668
Accrued workers’ compensation	98,683	94,291
Deferred income taxes	419,064	422,809
Other noncurrent liabilities	141,804	153,766
Total liabilities	6,754,617	6,761,569
Stockholders' equity	1,560,323	1,668,154
Total liabilities and stockholders’ equity	8,314,940	8,429,723
Parent/Issuer [Member]
Assets
Cash and cash equivalents	577,832	572,185	753,902	799,333
Short term investments	249,518	248,954
Receivables	10,184	9,656
Inventories	0	0
Other	82,714	89,211
Total current assets	920,248	920,006
Property, plant and equipment, net	9,860	10,470
Investment in subsidiaries	7,487,172	7,464,221
Intercompany receivables	0	0
Note receivable from Arch Western	675,000	675,000
Other	125,994	131,884
Total other assets	8,288,166	8,271,105
Total assets	9,218,274	9,201,581
Liabilities and Stockholders’ Equity
Accounts payable	12,377	23,394
Accrued expenses and other current liabilities	117,096	85,899
Current maturities of debt	24,645	27,625
Total current liabilities	154,118	136,918
Long-term debt	5,081,468	5,084,839
Intercompany payables	1,946,138	1,817,755
Note payable to Arch Coal	0
Asset retirement obligations	1,000	981
Accrued pension benefits	5,323	5,967
Accrued postretirement benefits other than pension	4,643	4,430
Accrued workers’ compensation	10,046	9,172
Deferred income taxes	419,064	422,809
Other noncurrent liabilities	36,530	50,919
Total liabilities	7,658,330	7,533,790
Stockholders' equity	1,559,944	1,667,791
Total liabilities and stockholders’ equity	9,218,274	9,201,581
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,430	150,358	100,353	100,418
Short term investments	0	0
Receivables	10,894	15,933
Inventories	240,113	190,253
Other	44,828	41,455
Total current assets	396,265	397,999
Property, plant and equipment, net	6,361,096	6,442,623
Investment in subsidiaries	0	0
Intercompany receivables	2,139,722	2,021,110
Note receivable from Arch Western	0	0
Other	285,227	300,058
Total other assets	2,424,949	2,321,168
Total assets	9,182,310	9,161,790
Liabilities and Stockholders’ Equity
Accounts payable	155,080	156,664
Accrued expenses and other current liabilities	208,275	220,017
Current maturities of debt	9,723	9,260
Total current liabilities	373,078	385,941
Long-term debt	36,514	38,646
Intercompany payables		0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	403,844	397,915
Accrued pension benefits	9,113	10,293
Accrued postretirement benefits other than pension	29,810	28,238
Accrued workers’ compensation	88,637	85,119
Deferred income taxes		0
Other noncurrent liabilities	105,069	102,461
Total liabilities	1,721,065	1,723,613
Stockholders' equity	7,461,245	7,438,177
Total liabilities and stockholders’ equity	9,182,310	9,161,790
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,710	11,688	11,506	11,348
Short term investments	0	0
Receivables	197,321	211,043
Inventories	0	0
Other	10,095	6,630
Total current assets	219,126	229,361
Property, plant and equipment, net		2
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	1,395	1,572
Total other assets	1,395	1,572
Total assets	220,521	230,935
Liabilities and Stockholders’ Equity
Accounts payable	23	55
Accrued expenses and other current liabilities	782	1,095
Current maturities of debt	0	0
Total current liabilities	805	1,150
Long-term debt	0	0
Intercompany payables	193,584	203,355
Note payable to Arch Coal	0	0
Asset retirement obligations	0	0
Accrued pension benefits	0	0
Accrued postretirement benefits other than pension	0	0
Accrued workers’ compensation	0	0
Deferred income taxes	0	0
Other noncurrent liabilities	205	386
Total liabilities	194,594	204,891
Stockholders' equity	25,927	26,044
Total liabilities and stockholders’ equity	220,521	230,935
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0	0
Short term investments	0	0
Receivables	(4,650)	(4,615)
Inventories	0	0
Other	0
Total current assets	(4,650)	(4,615)
Property, plant and equipment, net	379	363
Investment in subsidiaries	(7,487,172)	(7,464,221)
Intercompany receivables	(2,139,722)	(2,021,110)
Note receivable from Arch Western	(675,000)	(675,000)
Other		0
Total other assets	(10,301,894)	(10,160,331)
Total assets	(10,306,165)	(10,164,583)
Liabilities and Stockholders’ Equity
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,650)	(4,615)
Current maturities of debt	0	0
Total current liabilities	(4,650)	(4,615)
Long-term debt	0	0
Intercompany payables	(2,139,722)	(2,021,110)
Note payable to Arch Coal	(675,000)	(675,000)
Asset retirement obligations	0	0
Accrued pension benefits	0	0
Accrued postretirement benefits other than pension	0	0
Accrued workers’ compensation	0	0
Deferred income taxes	0	0
Other noncurrent liabilities	0	0
Total liabilities	(2,819,372)	(2,700,725)
Stockholders' equity	(7,486,793)	(7,463,858)
Total liabilities and stockholders’ equity	$ (10,306,165)	$ (10,164,583)