Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 730,119	$ 784,622	$ 117,770	$ 138,149
Restricted cash	2,290	3,453
Short term investments	248,414	234,305
Receivables	345,044	332,080
Inventories	368,240	365,424
Other	190,275	194,220
Total current assets	1,884,382	1,914,104
Property, plant and equipment, net	7,272,541	7,337,098
Investment in subsidiaries
Intercompany receivables
Note receivable from Arch Western
Other	763,221	755,575
Assets, Noncurrent	763,221	755,575
Total assets	9,920,144	10,006,777	10,165,118
Liabilities and Stockholders’ Equity
Accounts payable	229,269	224,418
Accrued expenses and other current liabilities	352,683	319,755
Current maturities of debt	28,306	32,896		32,896
Total current liabilities	610,258	577,069
Long-term debt	5,082,205	5,085,879		5,085,879
Note payable to Arch Coal
Asset retirement obligations	410,975	409,705
Accrued pension benefits	69,342	67,630
Accrued postretirement benefits other than pension	46,413	45,086
Accrued workers' compensation	81,039	81,629
Deferred income taxes	610,195	664,182
Other noncurrent liabilities	227,363	221,030
Total liabilities	7,137,790	7,152,210
Stockholders' equity	2,782,354	2,854,567
Total liabilities and stockholders' equity	9,920,144	10,006,777
Parent/Issuer [Member]
Assets
Cash and cash equivalents	597,308	671,313	35,196	66,542
Restricted cash	2,290	3,453
Short term investments	248,414	234,305
Receivables	50,179	49,281
Inventories
Other	101,003	106,786
Total current assets	999,194	1,065,138
Property, plant and equipment, net	26,716	27,476
Investment in subsidiaries	8,259,302	8,254,508
Intercompany receivables	(1,389,356)	(1,367,739)
Note receivable from Arch Western	675,000	675,000
Other	184,571	187,171
Assets, Noncurrent	7,729,517	7,748,940
Total assets	8,755,427	8,841,554
Liabilities and Stockholders’ Equity
Accounts payable	27,269	19,859
Accrued expenses and other current liabilities	105,059	65,293
Current maturities of debt	26,218	32,054
Total current liabilities	158,546	117,206
Long-term debt	5,059,611	5,061,925
Note payable to Arch Coal
Asset retirement obligations	1,672	1,646
Accrued pension benefits	34,595	33,456
Accrued postretirement benefits other than pension	14,486	13,953
Accrued workers' compensation	24,032	25,323
Deferred income taxes	610,195	664,182
Other noncurrent liabilities	69,936	69,296
Total liabilities	5,973,073	5,986,987
Stockholders' equity	2,782,354	2,854,567
Total liabilities and stockholders' equity	8,755,427	8,841,554
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	120,429	100,468	70,515	70,258
Restricted cash
Short term investments
Receivables	36,477	40,452
Inventories	368,240	365,424
Other	88,903	86,877
Total current assets	614,049	593,221
Property, plant and equipment, net	7,245,761	7,309,550
Investment in subsidiaries
Intercompany receivables	1,637,093	1,600,311
Note receivable from Arch Western
Other	578,560	568,314
Assets, Noncurrent	2,215,653	2,168,625
Total assets	10,075,463	10,071,396
Liabilities and Stockholders’ Equity
Accounts payable	201,925	204,370
Accrued expenses and other current liabilities	251,850	259,162
Current maturities of debt	2,088	842
Total current liabilities	455,863	464,374
Long-term debt	22,594	23,954
Note payable to Arch Coal	675,000	675,000
Asset retirement obligations	409,303	408,059
Accrued pension benefits	34,747	34,174
Accrued postretirement benefits other than pension	31,927	31,133
Accrued workers' compensation	57,007	56,306
Deferred income taxes
Other noncurrent liabilities	157,210	151,360
Total liabilities	1,843,651	1,844,360
Stockholders' equity	8,231,812	8,227,036
Total liabilities and stockholders' equity	10,075,463	10,071,396
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	12,382	12,841	12,059	1,349
Restricted cash
Short term investments
Receivables	263,123	247,171
Inventories
Other	369	557
Total current assets	275,874	260,569
Property, plant and equipment, net	64	72
Investment in subsidiaries
Intercompany receivables	(247,737)	(232,572)
Note receivable from Arch Western
Other	90	90
Assets, Noncurrent	(247,647)	(232,482)
Total assets	28,291	28,159
Liabilities and Stockholders’ Equity
Accounts payable	75	189
Accrued expenses and other current liabilities	509	124
Current maturities of debt
Total current liabilities	584	313
Long-term debt
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	217	374
Total liabilities	801	687
Stockholders' equity	27,490	27,472
Total liabilities and stockholders' equity	28,291	28,159
Eliminations [Member]
Assets
Cash and cash equivalents
Restricted cash
Short term investments
Receivables	(4,735)	(4,824)
Inventories
Other
Total current assets	(4,735)	(4,824)
Property, plant and equipment, net
Investment in subsidiaries	(8,259,302)	(8,254,508)
Intercompany receivables
Note receivable from Arch Western	(675,000)	(675,000)
Other
Assets, Noncurrent	(8,934,302)	(8,929,508)
Total assets	(8,939,037)	(8,934,332)
Liabilities and Stockholders’ Equity
Accounts payable
Accrued expenses and other current liabilities	(4,735)	(4,824)
Current maturities of debt
Total current liabilities	(4,735)	(4,824)
Long-term debt
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	(679,735)	(679,824)
Stockholders' equity	(8,259,302)	(8,254,508)
Total liabilities and stockholders' equity	$ (8,939,037)	$ (8,934,332)