Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Revenues	$ 1,063,538	$ 985,528	$ 2,103,189	$ 1,858,466
Costs, expenses and other
Cost of sales	881,259	715,590	1,732,130	1,369,274
Depreciation, depletion and amortization	132,868	97,236	272,834	180,773
Amortization of acquired sales contracts, net	(4,451)	1,262	(18,468)	7,206
Mine closure and asset impairment costs	525,762		525,762
Goodwill Impairment	115,791		115,791
Selling, general and administrative expenses	35,178	29,040	66,039	59,474
Change in fair value of coal derivatives and coal trading activities, net	(32,054)	2,672	(35,667)	888
Acquisition and transition costs related to ICG		48,666		48,666
Other operating (income) expense, net	(1,831)	(4,292)	(20,329)	(5,407)
Total operating expenses	1,652,522	890,174	2,638,092	1,660,874
Income (loss) from operations	(588,984)	95,354	(534,903)	197,592
Interest expense	(78,728)	(42,249)	(153,500)	(76,829)
Interest income	1,088	755	2,109	1,501
Interest expense, net	(77,640)	(41,494)	(151,391)	(75,328)
Bridge financing costs related to ICG		(49,490)		(49,490)
Net loss resulting from early retirement and refinancing of debt	(19,042)	(250)	(19,042)	(250)
Other nonoperating expense, Total	(19,042)	(49,740)	(19,042)	(49,740)
Income (loss) before income taxes	(685,666)	4,120	(705,336)	72,524
Provision for (benefit from) income taxes	(250,242)	(2,510)	(271,321)	10,020
Net income (loss)	(435,424)	6,630	(434,015)	62,504
Less: Net income attributable to noncontrolling interest	(65)	(318)	(268)	(591)
Net income (loss) attributable to Arch Coal, Inc.	(435,489)	6,312	(434,283)	61,913
Total comprehensive income (loss)	(438,473)	(1,998)	(429,764)	62,573
Parent/Issuer [Member]
Costs, expenses and other
Cost of sales	2,288	4,940	5,258	8,219
Depreciation, depletion and amortization	1,345	659	2,560	1,331
Selling, general and administrative expenses	21,774	18,600	40,418	38,936
Acquisition and transition costs related to ICG		48,666		48,666
Other operating (income) expense, net	6,472	(4,831)	3,360	(9,398)
Total operating expenses	31,879	68,034	51,596	87,754
Income from investment in subsidiaries	(553,007)	165,368	(475,692)	290,370
Income (loss) from operations	(584,886)	97,334	(527,288)	202,616
Interest expense	(89,740)	(48,033)	(171,837)	(88,654)
Interest income	6,309	4,309	11,138	8,052
Interest expense, net	(83,431)	(43,724)	(160,699)	(80,602)
Bridge financing costs related to ICG		(49,490)		(49,490)
Net loss resulting from early retirement and refinancing of debt	(17,349)		(17,349)
Other nonoperating expense, Total	(17,349)	(49,490)	(17,349)	(49,490)
Income (loss) before income taxes	(685,666)	4,120	(705,336)	72,524
Provision for (benefit from) income taxes	(250,242)	(2,510)	(272,902)	10,020
Net income (loss)	(435,424)	6,630	(432,434)	62,504
Less: Net income attributable to noncontrolling interest	(65)	(318)	(268)	(591)
Net income (loss) attributable to Arch Coal, Inc.	(435,489)	6,312	(432,702)	61,913
Total comprehensive income (loss)	(434,624)	20	(430,676)	66,392
Guarantor Subsidiaries [Member]
Revenues	558,400	419,895	1,057,129	758,429
Costs, expenses and other
Cost of sales	458,214	288,172	883,216	540,057
Depreciation, depletion and amortization	93,254	56,062	193,267	99,341
Amortization of acquired sales contracts, net	(4,482)	(4,340)	(17,683)	(4,340)
Mine closure and asset impairment costs	525,690		525,690
Goodwill Impairment	115,791		115,791
Selling, general and administrative expenses	2,700	3,171	4,686	5,053
Change in fair value of coal derivatives and coal trading activities, net	(32,054)	2,672	(35,667)	888
Other operating (income) expense, net	(35,930)	(29,248)	(73,629)	(56,702)
Total operating expenses	1,123,183	316,489	1,595,671	584,297
Income (loss) from operations	(564,783)	103,406	(538,542)	174,132
Interest expense	(1,198)	(2,135)	(2,377)	(2,849)
Interest income	159	136	405	432
Interest expense, net	(1,039)	(1,999)	(1,972)	(2,417)
Net loss resulting from early retirement and refinancing of debt		(250)		(250)
Other nonoperating expense, Total		(250)		(250)
Income (loss) before income taxes	(565,822)	101,157	(540,514)	171,465
Net income (loss)	(565,822)	101,157	(540,514)	171,465
Net income (loss) attributable to Arch Coal, Inc.	(565,822)	101,157	(540,514)	171,465
Total comprehensive income (loss)	(570,256)	102,245	(543,015)	172,495
Non-Guarantor Subsidiaries [Member]
Revenues	505,138	565,633	1,046,060	1,100,037
Costs, expenses and other
Cost of sales	442,387	447,707	890,442	871,030
Depreciation, depletion and amortization	38,270	40,516	77,009	80,101
Amortization of acquired sales contracts, net	31	5,602	(785)	11,546
Mine closure and asset impairment costs	72		72
Selling, general and administrative expenses	12,392	9,104	24,437	19,017
Other operating (income) expense, net	4,308	2,722	(350)	7,129
Total operating expenses	497,460	505,651	990,825	988,823
Income (loss) from operations	7,678	59,982	55,235	111,214
Interest expense	(8,809)	(10,962)	(20,152)	(21,944)
Interest income	15,639	15,191	31,432	29,635
Interest expense, net	6,830	4,229	11,280	7,691
Net loss resulting from early retirement and refinancing of debt	(1,693)		(1,693)
Other nonoperating expense, Total	(1,693)		(1,693)
Income (loss) before income taxes	12,815	64,211	64,822	118,905
Provision for (benefit from) income taxes			1,581
Net income (loss)	12,815	64,211	63,241	118,905
Net income (loss) attributable to Arch Coal, Inc.	12,815	64,211	63,241	118,905
Total comprehensive income (loss)	13,400	61,105	68,235	114,056
Eliminations [Member]
Costs, expenses and other
Cost of sales	(21,630)	(25,229)	(46,786)	(50,032)
Depreciation, depletion and amortization	(1)	(1)	(2)
Selling, general and administrative expenses	(1,688)	(1,835)	(3,502)	(3,532)
Other operating (income) expense, net	23,319	27,065	50,290	53,564
Income from investment in subsidiaries	553,007	(165,368)	475,692	(290,370)
Income (loss) from operations	553,007	(165,368)	475,692	(290,370)
Interest expense	21,019	18,881	40,866	36,618
Interest income	(21,019)	(18,881)	(40,866)	(36,618)
Income (loss) before income taxes	553,007	(165,368)	475,692	(290,370)
Net income (loss)	553,007	(165,368)	475,692	(290,370)
Net income (loss) attributable to Arch Coal, Inc.	553,007	(165,368)	475,692	(290,370)
Total comprehensive income (loss)	$ 553,007	$ (165,368)	$ 475,692	$ (290,370)