Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 400,205	$ 450,781	$ 494,788	$ 734,231
Short term investments	111,451	200,192
Restricted cash	81,563	97,542
Receivables	182,749	135,767
Inventories	159,410	196,720
Other	43,945	57,923
Total current assets	979,323	1,138,925
Property, plant and equipment, net	3,434,941	3,619,029
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	243,798	283,927
Total other assets	243,798	283,927
Total assets	4,658,062	5,041,881
Liabilities and Stockholders’ Deficit
Accounts payable	89,966	128,131
Accrued expenses and other current liabilities	223,780	329,450
Current maturities of debt	3,398	5,042,353
Total current liabilities	317,144	5,499,934
Long-term debt	30,037	30,953
Intercompany payables	0	0
Note payable to Arch Coal	0	0
Asset retirement obligations	394,699	396,659
Accrued pension benefits	23,716	27,373
Accrued postretirement benefits other than pension	87,123	99,810
Accrued workers’ compensation	119,828	112,270
Other noncurrent liabilities	65,824	119,171
Total liabilities not subject to compromise	1,038,371	6,286,170
Liabilities subject to compromise	5,295,785	0
Total liabilities	6,334,156	6,286,170
Stockholders’ equity (deficit)	(1,676,094)	(1,244,289)
Total liabilities and stockholders’ deficit	4,658,062	5,041,881
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0	0
Short term investments	0	0
Restricted cash	0	0
Receivables	(4,985)	(4,430)
Inventories	0	0
Other	0	0
Total current assets	(4,985)	(4,430)
Property, plant and equipment, net	436	413
Investment in subsidiaries	(5,958,449)	(4,887,905)
Intercompany receivables	(3,713,223)	(2,253,312)
Note receivable from Arch Western	(675,000)	(675,000)
Other	0	0
Total other assets	(10,346,672)	(7,816,217)
Total assets	(10,351,221)	(7,820,234)
Liabilities and Stockholders’ Deficit
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,982)	(4,430)
Current maturities of debt	0	0
Total current liabilities	(4,982)	(4,430)
Long-term debt	0	0
Intercompany payables	(3,713,223)	(2,253,313)
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
Other noncurrent liabilities	0	0
Total liabilities not subject to compromise	(4,393,205)
Total liabilities	(4,393,205)	(2,932,743)
Stockholders’ equity (deficit)	(5,958,016)	(4,887,491)
Total liabilities and stockholders’ deficit	(10,351,221)	(7,820,234)
Parent/Issuer [Member]
Assets
Cash and cash equivalents	388,338	337,646	362,643	572,185
Short term investments	111,451	200,192
Restricted cash	0	0
Receivables	15,750	12,463
Inventories	0	0
Other	21,333	18,160
Total current assets	536,872	568,461
Property, plant and equipment, net	5,644	7,747
Investment in subsidiaries	5,958,449	4,887,905
Intercompany receivables	0	0
Note receivable from Arch Western	675,000	675,000
Other	39,988	39,302
Total other assets	6,673,437	5,602,207
Total assets	7,215,953	6,178,415
Liabilities and Stockholders’ Deficit
Accounts payable	37,516	8,495
Accrued expenses and other current liabilities	69,723	162,268
Current maturities of debt	1,002	5,031,603
Total current liabilities	108,241	5,202,366
Long-term debt	0	0
Intercompany payables	3,474,361	2,043,308
Note payable to Arch Coal	0	0
Asset retirement obligations	1,062	1,005
Accrued pension benefits	4,034	12,390
Accrued postretirement benefits other than pension	71,722	79,826
Accrued workers’ compensation	17,620	24,247
Other noncurrent liabilities	26,718	59,976
Total liabilities not subject to compromise	3,703,758
Liabilities subject to compromise	5,188,722
Total liabilities	8,892,480	7,423,118
Stockholders’ equity (deficit)	(1,676,527)	(1,244,703)
Total liabilities and stockholders’ deficit	7,215,953	6,178,415
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	420	100,428	120,484	150,358
Short term investments	0	0
Restricted cash	0	0
Receivables	6,830	3,153
Inventories	159,410	196,720
Other	20,007	38,794
Total current assets	186,667	339,095
Property, plant and equipment, net	3,428,861	3,610,869
Investment in subsidiaries	0	0
Intercompany receivables	3,713,223	2,253,312
Note receivable from Arch Western	0	0
Other	203,265	243,806
Total other assets	3,916,488	2,497,118
Total assets	7,532,016	6,447,082
Liabilities and Stockholders’ Deficit
Accounts payable	52,410	119,633
Accrued expenses and other current liabilities	157,989	170,575
Current maturities of debt	2,396	10,750
Total current liabilities	212,795	300,958
Long-term debt	30,037	30,953
Intercompany payables	0	0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	393,637	395,654
Accrued pension benefits	19,682	14,983
Accrued postretirement benefits other than pension	15,401	19,984
Accrued workers’ compensation	102,208	88,023
Other noncurrent liabilities	38,824	58,847
Total liabilities not subject to compromise	1,487,584
Liabilities subject to compromise	107,063
Total liabilities	1,594,647	1,584,402
Stockholders’ equity (deficit)	5,937,369	4,862,680
Total liabilities and stockholders’ deficit	7,532,016	6,447,082
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,447	12,707	$ 11,661	$ 11,688
Short term investments	0	0
Restricted cash	81,563	97,542
Receivables	165,154	124,581
Inventories	0	0
Other	2,605	969
Total current assets	260,769	235,799
Property, plant and equipment, net	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	545	819
Total other assets	545	819
Total assets	261,314	236,618
Liabilities and Stockholders’ Deficit
Accounts payable	40	3
Accrued expenses and other current liabilities	1,050	1,037
Current maturities of debt	0	0
Total current liabilities	1,090	1,040
Long-term debt	0	0
Intercompany payables	238,862	210,005
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
Other noncurrent liabilities	282	348
Total liabilities not subject to compromise	240,234
Total liabilities	240,234	211,393
Stockholders’ equity (deficit)	21,080	25,225
Total liabilities and stockholders’ deficit	$ 261,314	$ 236,618