Supplemental Consolidating Financial Information (Schedule of Consolidating Balance Sheets) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Cash and cash equivalents	$ 450,781	$ 734,231	$ 911,099	$ 784,622
Short term investments	200,192	248,954
Restricted cash	97,542	5,678
Receivables	135,767	232,017
Inventories	196,720	190,253
Other	122,780	131,618
Total current assets	1,203,782	1,542,751
Property, plant and equipment, net	3,619,029	6,453,458
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Note receivable from Arch Western	0	0
Other assets	283,927	433,514
Total assets	5,106,738	8,429,723	8,990,193
Liabilities and Stockholders’ Equity
Accounts payable	128,131	180,113
Accrued expenses and other current liabilities	329,450	302,396
Current maturities of debt	5,107,210	36,885
Total current liabilities	5,564,791	519,394
Long-term debt	30,953	5,123,485
Intercompany payables	0	0
Note payable to Arch Coal	0	0
Asset retirement obligations	396,659	398,896
Accrued pension benefits	27,373	16,260
Accrued postretirement benefits other than pension	99,810	32,668
Accrued workers’ compensation	112,270	94,291
Deferred income taxes	0	422,809
Other noncurrent liabilities	119,171	153,766
Total liabilities	6,351,027	6,761,569
Total stockholders’ equity (deficit)	(1,244,289)	1,668,154	2,253,249	2,854,567
Total liabilities and stockholders’ equity (deficit)	5,106,738	8,429,723
Parent/Issuer [Member]
Assets
Cash and cash equivalents	337,646	572,185	799,333	671,313
Short term investments	200,192	248,954
Restricted cash	0	0
Receivables	12,463	9,656
Inventories	0	0
Other	83,017	89,211
Total current assets	633,318	920,006
Property, plant and equipment, net	7,747	10,470
Investment in subsidiaries	4,887,905	$ 7,464,221
Intercompany receivables	0
Note receivable from Arch Western	675,000	$ 675,000
Other assets	39,302	131,884
Total assets	6,243,272	9,201,581
Liabilities and Stockholders’ Equity
Accounts payable	8,495	23,394
Accrued expenses and other current liabilities	162,268	85,899
Current maturities of debt	5,096,460	27,625
Total current liabilities	5,267,223	136,918
Long-term debt	0	5,084,839
Intercompany payables	2,043,308	1,817,755
Note payable to Arch Coal	0	0
Asset retirement obligations	1,005	981
Accrued pension benefits	12,390	5,967
Accrued postretirement benefits other than pension	79,826	4,430
Accrued workers’ compensation	24,247	9,172
Deferred income taxes	0	422,809
Other noncurrent liabilities	59,976	50,919
Total liabilities	7,487,975	7,533,790
Total stockholders’ equity (deficit)	(1,244,703)	1,667,791
Total liabilities and stockholders’ equity (deficit)	6,243,272	9,201,581
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,428	150,358	100,418	100,468
Short term investments	0	0
Restricted cash	0	0
Receivables	3,153	15,933
Inventories	196,720	190,253
Other	38,794	41,455
Total current assets	339,095	397,999
Property, plant and equipment, net	3,610,869	6,442,623
Investment in subsidiaries	0	0
Intercompany receivables	2,253,312	2,021,110
Note receivable from Arch Western	0	0
Other assets	243,806	300,058
Total assets	6,447,082	9,161,790
Liabilities and Stockholders’ Equity
Accounts payable	119,633	156,664
Accrued expenses and other current liabilities	170,575	220,017
Current maturities of debt	10,750	9,260
Total current liabilities	300,958	385,941
Long-term debt	30,953	38,646
Intercompany payables	0	0
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	395,654	397,915
Accrued pension benefits	14,983	10,293
Accrued postretirement benefits other than pension	19,984	28,238
Accrued workers’ compensation	88,023	85,119
Deferred income taxes	0	0
Other noncurrent liabilities	58,847	102,461
Total liabilities	1,584,402	1,723,613
Total stockholders’ equity (deficit)	4,862,680	7,438,177
Total liabilities and stockholders’ equity (deficit)	6,447,082	9,161,790
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	12,707	11,688	11,348	12,841
Short term investments	0	0
Restricted cash	97,542	5,678
Receivables	124,581	211,043
Inventories	0
Other	969	952
Total current assets	235,799	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 assets	819	1,572
Total assets	236,618	230,935
Liabilities and Stockholders’ Equity
Accounts payable	3	55
Accrued expenses and other current liabilities	1,037	1,095
Current maturities of debt	0	0
Total current liabilities	1,040	1,150
Long-term debt	0	0
Intercompany payables	210,005	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	348	386
Total liabilities	211,393	204,891
Total stockholders’ equity (deficit)	25,225	26,044
Total liabilities and stockholders’ equity (deficit)	236,618	230,935
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	$ 0	$ 0
Short term investments	0	0
Restricted cash	0	0
Receivables	(4,430)	(4,615)
Inventories	0	0
Other	0	0
Total current assets	(4,430)	(4,615)
Property, plant and equipment, net	413	363
Investment in subsidiaries	(4,887,905)	(7,464,221)
Intercompany receivables	(2,253,312)	(2,021,110)
Note receivable from Arch Western	(675,000)	(675,000)
Other assets	0	0
Total assets	(7,820,234)	(10,164,583)
Liabilities and Stockholders’ Equity
Accounts payable	0	0
Accrued expenses and other current liabilities	(4,430)	(4,615)
Current maturities of debt	0	0
Total current liabilities	(4,430)	$ (4,615)
Long-term debt	0
Intercompany payables	(2,253,313)	$ (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	(2,932,743)	(2,700,725)
Total stockholders’ equity (deficit)	(4,887,491)	(7,463,858)
Total liabilities and stockholders’ equity (deficit)	$ (7,820,234)	$ (10,164,583)