Supplemental Consolidating Financial Information (Schedule of Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 911,099	$ 784,622	$ 138,149	$ 93,593
Restricted cash		3,453
Short term investments	248,414	234,305
Receivables	229,573	332,080
Inventories	264,161	365,424
Other	128,824	194,220
Total current assets	1,782,071	1,914,104
Property, plant and equipment, net	6,734,286	7,337,098
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	473,836	755,575
Assets, Noncurrent	473,836	755,575
Total assets	8,990,193	10,006,777	10,213,959
Liabilities and Stockholders’ Equity
Accounts payable	176,142	224,418
Accrued expenses and other current liabilities	278,587	319,755
Current maturities of debt	33,493	32,896
Total current liabilities	488,222	577,069
Long-term debt	5,118,002	5,085,879
Intercompany payables
Note payable to Arch Coal
Asset retirement obligations	402,713	409,705
Accrued pension benefits	7,111	67,630
Accrued postretirement benefits other than pension	39,255	45,086
Accrued workers’ compensation	78,062	81,629
Deferred income taxes	413,546	664,182
Other noncurrent liabilities	190,033	221,030
Total liabilities	6,736,944	7,152,210
Stockholders' equity	2,253,249	2,854,567	3,578,040	2,237,507
Total liabilities and stockholders’ equity	8,990,193	10,006,777
Parent/Issuer [Member]
Assets
Cash and cash equivalents	799,333	671,313	61,375	13,713
Restricted cash		3,453
Short term investments	248,414	234,305
Receivables	14,177	49,281
Inventories
Other	84,401	106,786
Total current assets	1,146,325	1,065,138
Property, plant and equipment, net	24,851	27,476
Investment in subsidiaries	7,741,589	8,254,508
Intercompany receivables
Note receivable from Arch Western	675,000	675,000
Other	162,287	187,171
Assets, Noncurrent	8,578,876	9,116,679
Total assets	9,750,052	10,209,293
Liabilities and Stockholders’ Equity
Accounts payable	17,781	19,859
Accrued expenses and other current liabilities	53,779	65,293
Current maturities of debt	28,882	32,054
Total current liabilities	100,442	117,206
Long-term debt	5,099,833	5,061,925
Intercompany payables	1,772,624	1,367,739
Note payable to Arch Coal
Asset retirement obligations	1,095	1,646
Accrued pension benefits	7,797	33,456
Accrued postretirement benefits other than pension	12,079	13,953
Accrued workers’ compensation	21,546	25,323
Deferred income taxes	413,546	664,182
Other noncurrent liabilities	67,841	69,296
Total liabilities	7,496,803	7,354,726
Stockholders' equity	2,253,249	2,854,567
Total liabilities and stockholders’ equity	9,750,052	10,209,293
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	100,418	100,468	75,425	79,880
Restricted cash
Short term investments
Receivables	23,018	40,452
Inventories	264,161	365,424
Other	43,617	86,877
Total current assets	431,214	593,221
Property, plant and equipment, net	6,709,398	7,309,550
Investment in subsidiaries
Intercompany receivables	1,953,719	1,600,311
Note receivable from Arch Western
Other	311,463	568,314
Assets, Noncurrent	2,265,182	2,168,625
Total assets	9,405,794	10,071,396
Liabilities and Stockholders’ Equity
Accounts payable	158,224	204,370
Accrued expenses and other current liabilities	228,664	259,162
Current maturities of debt	4,611	842
Total current liabilities	391,499	464,374
Long-term debt	18,169	23,954
Intercompany payables
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	401,618	408,059
Accrued pension benefits	(686)	34,174
Accrued postretirement benefits other than pension	27,176	31,133
Accrued workers’ compensation	56,516	56,306
Deferred income taxes
Other noncurrent liabilities	121,794	151,360
Total liabilities	1,691,086	1,844,360
Stockholders' equity	7,714,708	8,227,036
Total liabilities and stockholders’ equity	9,405,794	10,071,396
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	11,348	12,841	1,349
Restricted cash
Short term investments
Receivables	197,015	247,171
Inventories
Other	806	557
Total current assets	209,169	260,569
Property, plant and equipment, net	37	72
Investment in subsidiaries
Intercompany receivables	(181,095)
Note receivable from Arch Western
Other	86	90
Assets, Noncurrent	(181,009)	90
Total assets	28,197	260,731
Liabilities and Stockholders’ Equity
Accounts payable	137	189
Accrued expenses and other current liabilities	781	124
Current maturities of debt
Total current liabilities	918	313
Long-term debt
Intercompany payables		232,572
Note payable to Arch Coal
Asset retirement obligations
Accrued pension benefits
Accrued postretirement benefits other than pension
Accrued workers’ compensation
Deferred income taxes
Other noncurrent liabilities	398	374
Total liabilities	1,316	233,259
Stockholders' equity	26,881	27,472
Total liabilities and stockholders’ equity	28,197	260,731
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,637)	(4,824)
Inventories
Other
Total current assets	(4,637)	(4,824)
Property, plant and equipment, net
Investment in subsidiaries	(7,741,589)	(8,254,508)
Intercompany receivables	(1,772,624)	(1,600,311)
Note receivable from Arch Western	(675,000)	(675,000)
Other
Assets, Noncurrent	(10,189,213)	(10,529,819)
Total assets	(10,193,850)	(10,534,643)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,637)	(4,824)
Current maturities of debt
Total current liabilities	(4,637)	(4,824)
Long-term debt
Intercompany payables	(1,772,624)	(1,600,311)
Note payable to Arch Coal	(675,000)	(675,000)
Asset retirement obligations
Accrued pension benefits
Accrued postretirement benefits other than pension
Accrued workers’ compensation
Deferred income taxes
Other noncurrent liabilities
Total liabilities	(2,452,261)	(2,280,135)
Stockholders' equity	(7,741,589)	(8,254,508)
Total liabilities and stockholders’ equity	$ (10,193,850)	$ (10,534,643)