Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 395,398	$ 450,781	$ 689,972	$ 734,231
Short term investments	198,788	200,192
Restricted cash	109,650	97,542
Receivables	128,594	135,767
Inventories	195,987	196,720
Other	58,713	57,923
Total current assets	1,087,130	1,138,925
Property, plant and equipment, net	3,487,109	3,619,029
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other	281,171	283,927
Total other assets	281,171	283,927
Total assets	4,855,410	5,041,881
Liabilities and Stockholders’ Equity
Accounts payable	70,823	128,131
Accrued expenses and other current liabilities	102,590	329,450
Current maturities of debt	0	5,042,353
Total current liabilities	173,413	5,499,934
Long-term debt	0	30,953
Intercompany payables	0	0
Note payable to Arch Coal	0	0
Asset retirement obligations	401,569	396,659
Accrued pension benefits	20,646	27,373
Accrued postretirement benefits other than pension	89,779	99,810
Accrued workers’ compensation	113,782	112,270
Other noncurrent liabilities	18,023	119,171
Total liabilities not subject to compromise	817,212	6,286,170
Liabilities subject to compromise	5,487,326	0
Total liabilities	6,304,538	6,286,170
Stockholders' equity	(1,449,128)	(1,244,289)
Total liabilities and stockholders’ deficit	4,855,410	5,041,881
Parent/Issuer [Member]
Assets
Cash and cash equivalents	283,309	337,646	577,832	572,185
Short term investments	198,788	200,192
Restricted cash	0	0
Receivables	12,756	12,463
Inventories	0	0
Other	26,272	18,160
Total current assets	521,125	568,461
Property, plant and equipment, net	7,036	7,747
Investment in subsidiaries	4,729,318	4,887,905
Intercompany receivables	0	0
Note receivable from Arch Western	675,000	675,000
Other	38,578	39,302
Total other assets	5,442,896	5,602,207
Total assets	5,971,057	6,178,415
Liabilities and Stockholders’ Equity
Accounts payable	22,369	8,495
Accrued expenses and other current liabilities	26,357	162,268
Current maturities of debt	0	5,031,603
Total current liabilities	48,726	5,202,366
Long-term debt	0	0
Intercompany payables	2,062,135	2,043,308
Note payable to Arch Coal	0	0
Asset retirement obligations	1,024	1,005
Accrued pension benefits	4,115	12,390
Accrued postretirement benefits other than pension	75,226	79,826
Accrued workers’ compensation	9,500	24,247
Other noncurrent liabilities	6,502	59,976
Total liabilities not subject to compromise	2,207,228
Liabilities subject to compromise	5,213,376
Total liabilities	7,420,604	7,423,118
Stockholders' equity	(1,449,547)	(1,244,703)
Total liabilities and stockholders’ deficit	5,971,057	6,178,415
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,432	100,428	100,430	150,358
Short term investments	0	0
Restricted cash	0	0
Receivables	394	3,153
Inventories	195,987	196,720
Other	29,886	38,794
Total current assets	326,699	339,095
Property, plant and equipment, net	3,479,653	3,610,869
Investment in subsidiaries	0	0
Intercompany receivables	2,283,438	2,253,312
Note receivable from Arch Western	0	0
Other	240,872	243,806
Total other assets	2,524,310	2,497,118
Total assets	6,330,662	6,447,082
Liabilities and Stockholders’ Equity
Accounts payable	48,446	119,633
Accrued expenses and other current liabilities	80,396	170,575
Current maturities of debt	0	10,750
Total current liabilities	128,842	300,958
Long-term debt	0	30,953
Intercompany payables	0	0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	400,545	395,654
Accrued pension benefits	16,531	14,983
Accrued postretirement benefits other than pension	14,553	19,984
Accrued workers’ compensation	104,282	88,023
Other noncurrent liabilities	11,366	58,847
Total liabilities not subject to compromise	1,351,119
Liabilities subject to compromise	273,950
Total liabilities	1,625,069	1,584,402
Stockholders' equity	4,705,593	4,862,680
Total liabilities and stockholders’ deficit	6,330,662	6,447,082
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,657	12,707	11,710	11,688
Short term investments	0	0
Restricted cash	109,650	97,542
Receivables	120,359	124,581
Inventories	0	0
Other	2,555	969
Total current assets	244,221	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	1,721	819
Total other assets	1,721	819
Total assets	245,942	236,618
Liabilities and Stockholders’ Equity
Accounts payable	8	3
Accrued expenses and other current liabilities	752	1,037
Current maturities of debt	0	0
Total current liabilities	760	1,040
Long-term debt	0	0
Intercompany payables	221,302	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	155	348
Total liabilities not subject to compromise	222,217
Total liabilities	222,217	211,393
Stockholders' equity	23,725	25,225
Total liabilities and stockholders’ deficit	245,942	236,618
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	$ 0	$ 0
Short term investments	0	0
Restricted cash	0	0
Receivables	(4,915)	(4,430)
Inventories	0	0
Other	0	0
Total current assets	(4,915)	(4,430)
Property, plant and equipment, net	420	413
Investment in subsidiaries	(4,729,318)	(4,887,905)
Intercompany receivables	(2,283,438)	(2,253,312)
Note receivable from Arch Western	(675,000)	(675,000)
Other	0	0
Total other assets	(7,687,756)	(7,816,217)
Total assets	(7,692,251)	(7,820,234)
Liabilities and Stockholders’ Equity
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,915)	(4,430)
Current maturities of debt	0	0
Total current liabilities	(4,915)	(4,430)
Long-term debt	0	0
Intercompany payables	(2,283,437)	(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	(2,963,352)
Total liabilities	(2,963,352)	(2,932,743)
Stockholders' equity	(4,728,899)	(4,887,491)
Total liabilities and stockholders’ deficit	$ (7,692,251)	$ (7,820,234)