Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
REVENUES	$ 1,228,756	$ 1,198,673	$ 985,528	$ 872,938	$ 835,394	$ 874,705	$ 764,295	$ 711,874	$ 4,285,895	$ 3,186,268	$ 2,576,081
Costs, expenses and other
Cost of sales									3,267,910	2,395,812	2,070,715
Depreciation, depletion and amortization									466,587	365,066	301,608
Amortization of acquired sales contracts, net									(22,069)	35,606	19,623
Selling, general and administrative expenses									119,056	118,177	97,787
Change in fair value of coal derivatives and coal trading activities, net									(2,907)	8,924	(12,056)
Acquisition and transition costs									54,676		13,726
Gain on Knight Hawk transaction							(41,600)			(41,577)
Other operating (income) expense, net									(10,934)	(19,724)	(39,036)
Total operating expenses									3,872,319	2,862,284	2,452,367
Income from operations	139,728	76,256	95,354	102,238	86,938	98,347	106,499	32,200	413,576	323,984	123,714
Interest expense									(230,186)	(142,549)	(105,932)
Interest income									3,309	2,449	7,622
Interest expense, net									(226,877)	(140,100)	(98,310)
Bridge financing costs related to ICG									(49,490)
Net loss resulting from early retirement of debt									(1,958)	(6,776)
Other non-operating expense, Total									(51,448)	(6,776)
Income before income taxes									135,251	177,108	25,404
Provision for (benefit from) income taxes									(7,589)	17,714	(16,775)
Net income	71,215	9,121	6,630	55,874	48,031	46,859	66,274	(1,770)	142,840	159,394	42,179
Less: Net income attributable to noncontrolling interest									(1,157)	(537)	(10)
Net income attributable to Arch Coal, Inc.									141,683	158,857	42,169
Parent/Issuer [Member]
Costs, expenses and other
Cost of sales									22,925	11,526	7,481
Depreciation, depletion and amortization									2,883	2,933	3,678
Selling, general and administrative expenses									74,591	79,580	49,672
Acquisition and transition costs									54,676		13,726
Other operating (income) expense, net									(23,306)	(10,259)	(12,909)
Total operating expenses									131,769	83,780	61,648
Income from investment in subsidiaries									556,448	393,366	165,183
Income from operations									424,679	309,586	103,535
Interest expense									(256,220)	(143,606)	(92,371)
Interest income									16,282	11,128	14,240
Interest expense, net									(239,938)	(132,478)	(78,131)
Bridge financing costs related to ICG									(49,490)
Other non-operating expense, Total									(49,490)
Income before income taxes									135,251	177,108	25,404
Provision for (benefit from) income taxes									(7,589)	17,714	(16,775)
Net income									142,840	159,394	42,179
Less: Net income attributable to noncontrolling interest									(1,157)	(537)	(10)
Net income attributable to Arch Coal, Inc.									141,683	158,857	42,169
Guarantor Subsidiaries [Member]
REVENUES									2,024,168	1,137,980	924,692
Costs, expenses and other
Cost of sales									1,537,697	797,917	713,782
Depreciation, depletion and amortization									304,742	194,847	138,125
Amortization of acquired sales contracts, net									(41,527)
Selling, general and administrative expenses									13,860	7,355	7,504
Change in fair value of coal derivatives and coal trading activities, net									(2,907)	8,924	(12,056)
Gain on Knight Hawk transaction										(41,577)
Other operating (income) expense, net									(118,767)	(115,994)	(85,460)
Total operating expenses									1,693,098	851,472	761,895
Income from operations									331,070	286,508	162,797
Interest expense									(5,062)	(2,763)	(2,442)
Interest income									759	456	720
Interest expense, net									(4,303)	(2,307)	(1,722)
Net loss resulting from early retirement of debt									(1,958)
Other non-operating expense, Total									(1,958)
Income before income taxes									324,809	284,201	161,075
Net income									324,809	284,201	161,075
Net income attributable to Arch Coal, Inc.									324,809	284,201	161,075
Non-Guarantor Subsidiaries [Member]
REVENUES									2,261,727	2,048,288	1,651,389
Costs, expenses and other
Cost of sales									1,812,461	1,679,872	1,398,663
Depreciation, depletion and amortization									158,962	167,286	159,805
Amortization of acquired sales contracts, net									19,458	35,606	19,623
Selling, general and administrative expenses									37,839	38,496	46,563
Other operating (income) expense, net									18,732	5,772	4,170
Total operating expenses									2,047,452	1,927,032	1,628,824
Income from operations									214,275	121,256	22,565
Interest expense									(43,728)	(64,463)	(70,668)
Interest income									61,092	59,148	52,211
Interest expense, net									17,364	(5,315)	(18,457)
Net loss resulting from early retirement of debt										(6,776)
Other non-operating expense, Total										(6,776)
Income before income taxes									231,639	109,165	4,108
Net income									231,639	109,165	4,108
Net income attributable to Arch Coal, Inc.									231,639	109,165	4,108
Eliminations [Member]
Costs, expenses and other
Cost of sales									(105,173)	(93,503)	(49,211)
Selling, general and administrative expenses									(7,234)	(7,254)	(5,952)
Other operating (income) expense, net									112,407	100,757	55,163
Income from investment in subsidiaries									(556,448)	(393,366)	(165,183)
Income from operations									(556,448)	(393,366)	(165,183)
Interest expense									74,824	68,283	59,549
Interest income									(74,824)	(68,283)	(59,549)
Income before income taxes									(556,448)	(393,366)	(165,183)
Net income									(556,448)	(393,366)	(165,183)
Net income attributable to Arch Coal, Inc.									$ (556,448)	$ (393,366)	$ (165,183)