Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Income) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Revenue
Coal sales	$ 985,087	$ 764,295	$ 1,858,025	$ 1,476,169
Costs, expenses and other
Cost of coal sales	709,953	570,861	1,363,637	1,121,611
Depreciation, depletion and amortization	95,183	87,759	178,720	176,278
Amortization of acquired sales contracts, net	1,489	5,214	7,433	15,967
Selling, general and administrative expenses	29,039	35,344	59,474	62,510
Change in fair value of coal derivatives and coal trading activities, net	2,672	4,587	888	10,464
Acquisition and transition costs related to ICG	48,666		48,666
Gain on Knight Hawk transaction		(41,577)		(41,577)
Other operating (income) expense, net	(4,732)	(4,392)	(5,848)	(7,783)
Total operating expenses	882,270	657,796	1,652,970	1,337,470
Income from operations	102,817	106,499	205,055	138,699
Interest expense	(42,249)	(35,125)	(76,829)	(70,208)
Interest income	755	623	1,501	961
Interest expense, net	(41,494)	(34,502)	(75,328)	(69,247)
Bridge financing costs related to ICG	(49,490)		(49,490)
Net loss resulting from early retirement of ICG debt	(250)		(250)
Other non-operating expense, Total	(49,740)		(49,740)
Income before income taxes	11,583	71,997	79,987	69,452
Provision for income taxes	186	5,723	12,716	4,948
Net income	11,397	66,274	67,271	64,504
Less: Net income attributable to noncontrolling interest	(318)	(118)	(591)	(144)
Net income attributable to Arch Coal, Inc.	11,079	66,156	66,680	64,360
Parent/Issuer [Member]
Costs, expenses and other
Cost of coal sales	4,940	2,148	8,219	4,976
Depreciation, depletion and amortization	659	725	1,331	1,476
Selling, general and administrative expenses	18,600	23,251	38,936	41,894
Acquisition and transition costs related to ICG	48,666		48,666
Other operating (income) expense, net	(4,831)	(2,818)	(9,398)	(4,778)
Total operating expenses	68,034	23,306	87,754	43,568
Income from investment in subsidiaries	172,831	126,555	297,834	173,820
Income from operations	104,797	103,249	210,080	130,252
Interest expense	(47,824)	(33,524)	(88,445)	(64,955)
Interest income	4,100	2,272	7,842	4,155
Interest expense, net	(43,724)	(31,252)	(80,603)	(60,800)
Bridge financing costs related to ICG	(49,490)		(49,490)
Other non-operating expense, Total	(49,490)		(49,490)
Income before income taxes	11,583	71,997	79,987	69,452
Provision for income taxes	186	5,723	12,716	4,948
Net income	11,397	66,274	67,271	64,504
Less: Net income attributable to noncontrolling interest	(318)	(118)	(591)	(144)
Net income attributable to Arch Coal, Inc.	11,079	66,156	66,680	64,360
Guarantor Subsidiaries [Member]
Revenue
Coal sales	419,454	282,817	757,988	521,845
Costs, expenses and other
Cost of coal sales	282,535	193,243	534,420	361,961
Depreciation, depletion and amortization	54,008	46,941	97,288	90,659
Amortization of acquired sales contracts, net	(4,113)		(4,113)
Selling, general and administrative expenses	3,170	1,836	5,053	3,642
Change in fair value of coal derivatives and coal trading activities, net	2,672	4,587	888	10,464
Gain on Knight Hawk transaction		(41,577)		(41,577)
Other operating (income) expense, net	(29,687)	(26,202)	(57,143)	(48,924)
Total operating expenses	308,585	178,828	576,393	376,225
Income from operations	110,869	103,989	181,595	145,620
Interest expense	(1,924)	(780)	(2,638)	(1,359)
Interest income	135	75	432	164
Interest expense, net	(1,789)	(705)	(2,206)	(1,195)
Net loss resulting from early retirement of ICG debt	(250)		(250)
Other non-operating expense, Total	(250)		(250)
Income before income taxes	108,830	103,284	179,139	144,425
Net income	108,830	103,284	179,139	144,425
Net income attributable to Arch Coal, Inc.	108,830	103,284	179,139	144,425
Non-Guarantor Subsidiaries [Member]
Revenue
Coal sales	565,633	481,478	1,100,037	954,324
Costs, expenses and other
Cost of coal sales	447,707	396,568	871,030	794,079
Depreciation, depletion and amortization	40,516	40,093	80,101	84,143
Amortization of acquired sales contracts, net	5,602	5,214	11,546	15,967
Selling, general and administrative expenses	9,104	12,006	19,017	20,409
Other operating (income) expense, net	2,722	1,781	7,129	3,079
Total operating expenses	505,651	455,662	988,823	917,677
Income from operations	59,982	25,816	111,214	36,647
Interest expense	(10,962)	(18,387)	(21,944)	(36,503)
Interest income	14,981	15,842	29,425	29,251
Interest expense, net	4,019	(2,545)	7,481	(7,252)
Income before income taxes	64,001	23,271	118,695	29,395
Net income	64,001	23,271	118,695	29,395
Net income attributable to Arch Coal, Inc.	64,001	23,271	118,695	29,395
Eliminations [Member]
Costs, expenses and other
Cost of coal sales	(25,229)	(21,098)	(50,032)	(39,405)
Selling, general and administrative expenses	(1,835)	(1,749)	(3,532)	(3,435)
Other operating (income) expense, net	27,064	22,847	53,564	42,840
Income from investment in subsidiaries	(172,831)	(126,555)	(297,834)	(173,820)
Income from operations	(172,831)	(126,555)	(297,834)	(173,820)
Interest expense	18,461	17,566	36,198	32,609
Interest income	(18,461)	(17,566)	(36,198)	(32,609)
Income before income taxes	(172,831)	(126,555)	(297,834)	(173,820)
Net income	(172,831)	(126,555)	(297,834)	(173,820)
Net income attributable to Arch Coal, Inc.	$ (172,831)	$ (126,555)	$ (297,834)	$ (173,820)