Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Revenues	$ 1,039,651	$ 872,938
Costs, expenses and other
Cost of sales	850,871	653,684
Depreciation, depletion and amortization	139,966	83,537
Amortization of acquired sales contracts, net	(14,017)	5,944
Selling, general and administrative expenses	30,861	30,435
Change in fair value of coal derivatives and coal trading activities, net	(3,613)	(1,784)
Other operating (income) expense, net	(18,498)	(1,116)
Total operating expenses	985,570	770,700
Income from operations	54,081	102,238
Interest expense	(74,772)	(34,580)
Interest income	1,021	746
Interest expense, net	(73,751)	(33,834)
Income before income taxes	(19,670)	68,404
Benefit from income taxes	(21,079)	12,530
Net income	1,409	55,874
Less: Net income attributable to noncontrolling interest	(203)	(273)
Net income attributable to Arch Coal, Inc.	1,206	55,601
Total comprehensive income	8,709	64,571
Total comprehensive income	8,709	64,571
Parent/Issuer [Member]
Costs, expenses and other
Cost of sales	2,964	3,279
Depreciation, depletion and amortization	1,217	672
Selling, general and administrative expenses	18,642	20,336
Other operating (income) expense, net	(3,110)	(4,567)
Total operating expenses	19,713	19,720
Income from investment in subsidiaries	77,312	125,003
Income from operations	57,599	105,283
Interest expense	(82,096)	(40,621)
Interest income	4,827	3,742
Interest expense, net	(77,269)	(36,879)
Income before income taxes	(19,670)	68,404
Benefit from income taxes	(22,660)	12,530
Net income	2,990	55,874
Less: Net income attributable to noncontrolling interest	(203)	(273)
Net income attributable to Arch Coal, Inc.	2,787	55,601
Total comprehensive income	3,948	66,372
Total comprehensive income	3,948	66,372
Guarantor Subsidiaries [Member]
Revenues	498,728	338,533
Costs, expenses and other
Cost of sales	425,009	251,884
Depreciation, depletion and amortization	100,006	43,277
Amortization of acquired sales contracts, net	(13,201)
Selling, general and administrative expenses	1,987	1,883
Change in fair value of coal derivatives and coal trading activities, net	(3,613)	(1,784)
Other operating (income) expense, net	(37,700)	(27,456)
Total operating expenses	472,488	267,804
Income from operations	26,240	70,729
Interest expense	(1,179)	(714)
Interest income	248	296
Interest expense, net	(931)	(418)
Income before income taxes	25,309	70,311
Net income	25,309	70,311
Net income attributable to Arch Coal, Inc.	25,309	70,311
Total comprehensive income	27,242	70,253
Total comprehensive income	27,242	70,253
Non-Guarantor Subsidiaries [Member]
Revenues	540,923	534,405
Costs, expenses and other
Cost of sales	448,054	423,323
Depreciation, depletion and amortization	38,744	39,588
Amortization of acquired sales contracts, net	(816)	5,944
Selling, general and administrative expenses	12,046	9,913
Other operating (income) expense, net	(4,659)	4,408
Total operating expenses	493,369	483,176
Income from operations	47,554	51,229
Interest expense	(11,344)	(10,982)
Interest income	15,793	14,445
Interest expense, net	4,449	3,463
Income before income taxes	52,003	54,692
Benefit from income taxes	1,581
Net income	50,422	54,692
Net income attributable to Arch Coal, Inc.	50,422	54,692
Total comprehensive income	54,831	52,949
Total comprehensive income	54,831	52,949
Eliminations [Member]
Costs, expenses and other
Cost of sales	(25,156)	(24,802)
Depreciation, depletion and amortization	(1)
Selling, general and administrative expenses	(1,814)	(1,697)
Other operating (income) expense, net	26,971	26,499
Income from investment in subsidiaries	(77,312)	(125,003)
Income from operations	(77,312)	(125,003)
Interest expense	19,847	17,737
Interest income	(19,847)	(17,737)
Income before income taxes	(77,312)	(125,003)
Net income	(77,312)	(125,003)
Net income attributable to Arch Coal, Inc.	(77,312)	(125,003)
Total comprehensive income	(77,312)	(125,003)
Total comprehensive income	$ (77,312)	$ (125,003)