Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Income) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Revenues	$ 1,198,673	$ 874,705	$ 3,057,139	$ 2,350,874
Costs, expenses and other
Cost of sales	952,850	651,853	2,322,124	1,773,464
Depreciation, depletion and amortization	123,026	92,857	301,746	269,135
Amortization of acquired sales contracts, net	(12,186)	10,038	(4,753)	26,005
Selling, general and administrative expenses	33,276	26,999	92,750	89,509
Change in fair value of coal derivatives and coal trading activities, net	8,360	1,832	9,248	12,296
Acquisition and transition costs related to ICG	4,694		53,360
Gain on Knight Hawk transaction				(41,577)
Other operating (income) expense, net	(3,613)	(7,221)	(9,019)	(15,004)
Total operating expenses	1,106,407	776,358	2,765,456	2,113,828
Income from operations	92,266	98,347	291,683	237,046
Interest expense	(77,694)	(37,698)	(154,523)	(107,906)
Interest income	840	927	2,341	1,888
Interest expense, net	(76,854)	(36,771)	(152,182)	(106,018)
Bridge financing costs related to ICG			(49,490)
Net loss resulting from early retirement of debt	(1,708)	(6,776)	(1,958)	(6,776)
Other non-operating expense, Total	(1,708)	(6,776)	(51,448)	(6,776)
Income before income taxes	13,704	54,800	88,053	124,252
Provision for (benefit from) income taxes	(5,583)	7,941	5,103	12,889
Net income	19,287	46,859	82,950	111,363
Less: Net income attributable to noncontrolling interest	(231)	(181)	(822)	(325)
Net income attributable to Arch Coal, Inc.	19,056	46,678	82,128	111,038
Parent/Issuer [Member]
Costs, expenses and other
Cost of sales	7,442	3,578	15,661	8,554
Depreciation, depletion and amortization	718	794	2,050	2,270
Selling, general and administrative expenses	20,263	18,245	59,198	60,139
Acquisition and transition costs related to ICG	4,694		53,360
Other operating (income) expense, net	(6,063)	(3,346)	(15,461)	(8,124)
Total operating expenses	27,054	19,271	114,808	62,839
Income from investment in subsidiaries	121,210	108,974	413,406	282,794
Income from operations	94,156	89,703	298,598	219,955
Interest expense	(84,045)	(38,041)	(172,700)	(102,996)
Interest income	3,593	3,138	11,645	7,293
Interest expense, net	(80,452)	(34,903)	(161,055)	(95,703)
Bridge financing costs related to ICG			(49,490)
Other non-operating expense, Total			(49,490)
Income before income taxes	13,704	54,800	88,053	124,252
Provision for (benefit from) income taxes	(5,583)	7,941	5,103	12,889
Net income	19,287	46,859	82,950	111,363
Less: Net income attributable to noncontrolling interest	(231)	(181)	(822)	(325)
Net income attributable to Arch Coal, Inc.	19,056	46,678	82,128	111,038
Guarantor Subsidiaries [Member]
Revenues	651,326	324,507	1,409,755	846,351
Costs, expenses and other
Cost of sales	522,636	229,731	1,062,692	591,692
Depreciation, depletion and amortization	83,775	52,302	181,059	142,961
Amortization of acquired sales contracts, net	(15,989)		(20,102)
Selling, general and administrative expenses	5,058	1,858	10,112	5,500
Change in fair value of coal derivatives and coal trading activities, net	8,360	1,832	9,248	12,296
Gain on Knight Hawk transaction				(41,577)
Other operating (income) expense, net	(30,509)	(35,107)	(87,210)	(84,031)
Total operating expenses	573,331	250,616	1,155,799	626,841
Income from operations	77,995	73,891	253,956	219,510
Interest expense	(942)	(745)	(3,793)	(2,104)
Interest income	108	119	540	283
Interest expense, net	(834)	(626)	(3,253)	(1,821)
Net loss resulting from early retirement of debt	(1,708)		(1,958)
Other non-operating expense, Total	(1,708)		(1,958)
Income before income taxes	75,453	73,265	248,745	217,689
Net income	75,453	73,265	248,745	217,689
Net income attributable to Arch Coal, Inc.	75,453	73,265	248,745	217,689
Non-Guarantor Subsidiaries [Member]
Revenues	547,347	550,198	1,647,384	1,504,523
Costs, expenses and other
Cost of sales	448,680	445,814	1,319,711	1,239,893
Depreciation, depletion and amortization	38,533	39,761	118,637	123,904
Amortization of acquired sales contracts, net	3,803	10,038	15,349	26,005
Selling, general and administrative expenses	9,751	8,818	28,768	29,226
Other operating (income) expense, net	5,255	2,040	12,384	5,120
Total operating expenses	506,022	506,471	1,494,849	1,424,148
Income from operations	41,325	43,727	152,535	80,375
Interest expense	(10,657)	(16,738)	(32,599)	(53,241)
Interest income	15,089	15,496	44,725	44,747
Interest expense, net	4,432	(1,242)	12,126	(8,494)
Net loss resulting from early retirement of debt		(6,776)		(6,776)
Other non-operating expense, Total		(6,776)		(6,776)
Income before income taxes	45,757	35,709	164,661	65,105
Net income	45,757	35,709	164,661	65,105
Net income attributable to Arch Coal, Inc.	45,757	35,709	164,661	65,105
Eliminations [Member]
Costs, expenses and other
Cost of sales	(25,908)	(27,270)	(75,940)	(66,675)
Selling, general and administrative expenses	(1,796)	(1,922)	(5,328)	(5,356)
Other operating (income) expense, net	27,704	29,192	81,268	72,031
Income from investment in subsidiaries	(121,210)	(108,974)	(413,406)	(282,794)
Income from operations	(121,210)	(108,974)	(413,406)	(282,794)
Interest expense	17,950	17,826	54,569	50,435
Interest income	(17,950)	(17,826)	(54,569)	(50,435)
Income before income taxes	(121,210)	(108,974)	(413,406)	(282,794)
Net income	(121,210)	(108,974)	(413,406)	(282,794)
Net income attributable to Arch Coal, Inc.	$ (121,210)	$ (108,974)	$ (413,406)	$ (282,794)