Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets
Cash and cash equivalents	$ 494,788	$ 734,231	$ 795,934	$ 795,934	$ 911,099
Short term investments	199,731	248,954
Restricted cash	50,409	5,678
Receivables	239,409	232,017
Inventories	239,035	190,253
Other	95,947	131,618
Total current assets	1,319,319	1,542,751
Property, plant and equipment, net	4,173,038	6,453,458
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	355,640	433,514
Total other assets	355,640	433,514
Total assets	5,847,997	8,429,723
Liabilities and Stockholders’ Equity
Accounts payable	152,229	180,113
Accrued expenses and other current liabilities	310,747	302,396
Current maturities of debt	32,237	36,885
Total current liabilities	495,213	519,394
Long-term debt	5,108,492	5,123,485
Intercompany payables	0	0
Note payable to Arch Coal	0	0
Asset retirement obligations	414,194	398,896
Accrued pension benefits	12,061	16,260
Accrued postretirement benefits other than pension	33,966	32,668
Accrued workers’ compensation	95,905	94,291
Deferred income taxes	44,806	422,809
Other noncurrent liabilities	248,800	153,766
Total liabilities	6,453,437	6,761,569
Stockholders' equity	(605,440)	1,668,154
Total liabilities and stockholders’ deficit	5,847,997	8,429,723
Parent/Issuer [Member]
Assets
Cash and cash equivalents	362,643	572,185	633,822		799,333
Short term investments	199,731	248,954
Restricted cash	0	0
Receivables	14,307	9,656
Inventories	0	0
Other	50,449	89,211
Total current assets	627,130	920,006
Property, plant and equipment, net	8,447	10,470
Investment in subsidiaries	5,421,088	7,464,221
Intercompany receivables	0	0
Note receivable from Arch Western	675,000	675,000
Other	99,939	131,884
Total other assets	6,196,027	8,271,105
Total assets	6,831,604	9,201,581
Liabilities and Stockholders’ Equity
Accounts payable	13,566	23,394
Accrued expenses and other current liabilities	118,662	85,899
Current maturities of debt	21,810	27,625
Total current liabilities	154,038	136,918
Long-term debt	5,074,812	5,084,839
Intercompany payables	1,971,290	1,817,755
Note payable to Arch Coal	0	0
Asset retirement obligations	1,037	981
Accrued pension benefits	4,247	5,967
Accrued postretirement benefits other than pension	4,234	4,430
Accrued workers’ compensation	10,291	9,172
Deferred income taxes	44,806	422,809
Other noncurrent liabilities	172,691	50,919
Total liabilities	7,437,446	7,533,790
Stockholders' equity	(605,842)	1,667,791
Total liabilities and stockholders’ deficit	6,831,604	9,201,581
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	120,484	150,358	150,359		100,418
Short term investments	0	0
Restricted cash	0	0
Receivables	12,502	15,933
Inventories	239,035	190,253
Other	44,565	41,455
Total current assets	416,586	397,999
Property, plant and equipment, net	4,164,189	6,442,623
Investment in subsidiaries	0	0
Intercompany receivables	2,225,149	2,021,110
Note receivable from Arch Western	0	0
Other	254,712	300,058
Total other assets	2,479,861	2,321,168
Total assets	7,060,636	9,161,790
Liabilities and Stockholders’ Equity
Accounts payable	138,624	156,664
Accrued expenses and other current liabilities	195,577	220,017
Current maturities of debt	10,427	9,260
Total current liabilities	344,628	385,941
Long-term debt	33,680	38,646
Intercompany payables	0	0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	413,157	397,915
Accrued pension benefits	7,814	10,293
Accrued postretirement benefits other than pension	29,732	28,238
Accrued workers’ compensation	85,614	85,119
Deferred income taxes	0	0
Other noncurrent liabilities	75,810	102,461
Total liabilities	1,665,435	1,723,613
Stockholders' equity	5,395,201	7,438,177
Total liabilities and stockholders’ deficit	7,060,636	9,161,790
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,661	11,688	11,753		11,348
Short term investments	0	0
Restricted cash	50,409	5,678
Receivables	216,959	211,043
Inventories	0	0
Other	933	952
Total current assets	279,962	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	989	1,572
Total other assets	989	1,572
Total assets	280,951	230,935
Liabilities and Stockholders’ Equity
Accounts payable	39	55
Accrued expenses and other current liabilities	867	1,095
Current maturities of debt	0	0
Total current liabilities	906	1,150
Long-term debt	0	0
Intercompany payables	253,859	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	299	386
Total liabilities	255,064	204,891
Stockholders' equity	25,887	26,044
Total liabilities and stockholders’ deficit	280,951	230,935
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	$ 0		$ 0
Short term investments	0	0
Restricted cash	0	0
Receivables	(4,359)	(4,615)
Inventories	0	0
Other	0	0
Total current assets	(4,359)	(4,615)
Property, plant and equipment, net	402	363
Investment in subsidiaries	(5,421,088)	(7,464,221)
Intercompany receivables	(2,225,149)	(2,021,110)
Note receivable from Arch Western	(675,000)	(675,000)
Other	0	0
Total other assets	(8,321,237)	(10,160,331)
Total assets	(8,325,194)	(10,164,583)
Liabilities and Stockholders’ Equity
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,359)	(4,615)
Current maturities of debt	0	0
Total current liabilities	(4,359)	(4,615)
Long-term debt	0	0
Intercompany payables	(2,225,149)	(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	(2,904,508)	(2,700,725)
Stockholders' equity	(5,420,686)	(7,463,858)
Total liabilities and stockholders’ deficit	$ (8,325,194)	$ (10,164,583)