Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$ 439,655	$ 734,231	$ 740,154	$ 740,154	$ 911,099
Short term investments	249,754	248,954
Restricted cash	43,563	5,678
Receivables	219,541	232,017
Inventories	223,929	190,253
Other	120,479	131,618
Total current assets	1,296,921	1,542,751
Property, plant and equipment, net	6,341,026	6,453,458
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	398,408	433,514
Total other assets	398,408	433,514
Total assets	8,036,355	8,429,723
Liabilities and Stockholders’ Equity
Accounts payable	156,725	180,113
Accrued expenses and other current liabilities	262,958	302,396
Current maturities of debt	31,763	36,885
Total current liabilities	451,446	519,394
Long-term debt	5,114,581	5,123,485
Intercompany payables	0	0
Note payable to Arch Coal	0	0
Asset retirement obligations	409,435	398,896
Accrued pension benefits	13,580	16,260
Accrued postretirement benefits other than pension	34,176	32,668
Accrued workers’ compensation	97,489	94,291
Deferred income taxes	411,930	422,809
Other noncurrent liabilities	109,693	153,766
Total liabilities	6,642,330	6,761,569
Stockholders' equity	1,394,025	1,668,154
Total liabilities and stockholders’ equity	8,036,355	8,429,723
Parent/Issuer [Member]
Assets
Cash and cash equivalents	367,668	572,185	628,251		799,333
Short term investments	249,754	248,954
Restricted cash	0	0
Receivables	9,525	9,656
Inventories	0	0
Other	80,295	89,211
Total current assets	707,242	920,006
Property, plant and equipment, net	9,211	10,470
Investment in subsidiaries	7,448,526	7,464,221
Intercompany receivables	0	0
Note receivable from Arch Western	675,000	675,000
Other	119,670	131,884
Total other assets	8,243,196	8,271,105
Total assets	8,959,649	9,201,581
Liabilities and Stockholders’ Equity
Accounts payable	12,096	23,394
Accrued expenses and other current liabilities	59,336	85,899
Current maturities of debt	21,651	27,625
Total current liabilities	93,083	136,918
Long-term debt	5,078,125	5,084,839
Intercompany payables	1,930,704	1,817,755
Note payable to Arch Coal	0	0
Asset retirement obligations	1,018	981
Accrued pension benefits	5,023	5,967
Accrued postretirement benefits other than pension	4,298	4,430
Accrued workers’ compensation	10,714	9,172
Deferred income taxes	411,930	422,809
Other noncurrent liabilities	31,120	50,919
Total liabilities	7,566,015	7,533,790
Stockholders' equity	1,393,634	1,667,791
Total liabilities and stockholders’ equity	8,959,649	9,201,581
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	60,568	150,358	100,360		100,418
Short term investments	0	0
Restricted cash	0	0
Receivables	10,352	15,933
Inventories	223,929	190,253
Other	39,160	41,455
Total current assets	334,009	397,999
Property, plant and equipment, net	6,331,424	6,442,623
Investment in subsidiaries	0	0
Intercompany receivables	2,164,803	2,021,110
Note receivable from Arch Western	0	0
Other	277,555	300,058
Total other assets	2,442,358	2,321,168
Total assets	9,107,791	9,161,790
Liabilities and Stockholders’ Equity
Accounts payable	144,588	156,664
Accrued expenses and other current liabilities	207,265	220,017
Current maturities of debt	10,112	9,260
Total current liabilities	361,965	385,941
Long-term debt	36,456	38,646
Intercompany payables	0	0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	408,417	397,915
Accrued pension benefits	8,557	10,293
Accrued postretirement benefits other than pension	29,878	28,238
Accrued workers’ compensation	86,775	85,119
Deferred income taxes	0	0
Other noncurrent liabilities	78,332	102,461
Total liabilities	1,685,380	1,723,613
Stockholders' equity	7,422,411	7,438,177
Total liabilities and stockholders’ equity	9,107,791	9,161,790
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,419	11,688	11,543		11,348
Short term investments	0	0
Restricted cash	43,563	5,678
Receivables	204,002	211,043
Inventories	0	0
Other	1,024	952
Total current assets	260,008	229,361
Property, plant and equipment, net	0	2
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	1,183	1,572
Total other assets	1,183	1,572
Total assets	261,191	230,935
Liabilities and Stockholders’ Equity
Accounts payable	41	55
Accrued expenses and other current liabilities	695	1,095
Current maturities of debt	0	0
Total current liabilities	736	1,150
Long-term debt	0	0
Intercompany payables	234,099	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	241	386
Total liabilities	235,076	204,891
Stockholders' equity	26,115	26,044
Total liabilities and stockholders’ equity	261,191	230,935
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	$ 0		$ 0
Short term investments	0	0
Restricted cash	0	0
Receivables	(4,338)	(4,615)
Inventories	0	0
Other	0	0
Total current assets	(4,338)	(4,615)
Property, plant and equipment, net	391	363
Investment in subsidiaries	(7,448,526)	(7,464,221)
Intercompany receivables	(2,164,803)	(2,021,110)
Note receivable from Arch Western	(675,000)	(675,000)
Other	0	0
Total other assets	(10,288,329)	(10,160,331)
Total assets	(10,292,276)	(10,164,583)
Liabilities and Stockholders’ Equity
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,338)	(4,615)
Current maturities of debt	0	0
Total current liabilities	(4,338)	(4,615)
Long-term debt	0	0
Intercompany payables	(2,164,803)	(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	(2,844,141)	(2,700,725)
Stockholders' equity	(7,448,135)	(7,463,858)
Total liabilities and stockholders’ equity	$ (10,292,276)	$ (10,164,583)