Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Condensed Financial Statement Line Items
Revenues	$ 1,087,618	$ 1,198,673	$ 3,190,807	$ 3,057,139
Costs, expenses and other
Cost of sales	896,809	952,850	2,628,939	2,322,124
Depreciation, depletion and amortization	126,838	139,547	399,672	320,320
Amortization of acquired sales contracts, net	(4,093)	(12,698)	(22,561)	(5,492)
Legal contingencies	(79,532)		(79,532)
Mine closure and asset impairment costs	(2,194)		523,568	7,316
Goodwill impairment			115,791
Selling, general and administrative expenses	33,266	33,275	99,305	92,749
Change in fair value of coal derivatives and coal trading activities, net	5,840	8,360	(29,827)	9,248
Acquisition and transition costs related to ICG		4,694		46,044
Other operating (income) expense, net	(25,276)	(3,611)	(45,605)	(9,018)
Total operating expenses	951,658	1,122,417	3,589,750	2,783,291
Income from investment in subsidiaries
Income (loss) from operations	135,960	76,256	(398,943)	273,848
Interest expense, net:
Interest expense	(75,710)	(77,694)	(229,210)	(154,523)
Interest income	1,459	840	3,568	2,341
Interest expense, net	(74,251)	(76,854)	(225,642)	(152,182)
Other non-operating expense
Bridge financing costs related to ICG				(49,490)
Net loss resulting from early retirement and refinancing of debt		(1,708)	(19,042)	(1,958)
Other nonoperating expense, Total		(1,708)	(19,042)	(51,448)
Income (loss) before income taxes	61,709	(2,306)	(643,627)	70,218
Provision for (benefit from) income taxes	15,958	(11,427)	(255,363)	(1,407)
Net income (loss)	45,751	9,121	(388,264)	71,625
Less: Net income attributable to noncontrolling interest		(231)	(268)	(822)
Net income (loss) attributable to Arch Coal, Inc.	45,751	8,890	(388,532)	70,803
Total comprehensive income (loss)	44,299	1,692	(385,524)	64,262
Parent/Issuer [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales	2,434	7,439	7,691	15,658
Depreciation, depletion and amortization	1,400	720	3,960	2,051
Amortization of acquired sales contracts, net
Legal contingencies
Mine closure and asset impairment costs
Goodwill impairment
Selling, general and administrative expenses	22,050	20,261	62,468	59,197
Change in fair value of coal derivatives and coal trading activities, net
Acquisition and transition costs related to ICG		4,694		46,044
Other operating (income) expense, net	(11,051)	(6,062)	(7,692)	(15,460)
Total operating expenses	14,833	27,052	66,427	107,490
Income from investment in subsidiaries	159,653	105,198	(316,041)	388,253
Income (loss) from operations	144,820	78,146	(382,468)	280,763
Interest expense, net:
Interest expense	(91,589)	(84,044)	(263,428)	(172,699)
Interest income	8,478	3,592	19,615	11,644
Interest expense, net	(83,111)	(80,452)	(243,813)	(161,055)
Other non-operating expense
Bridge financing costs related to ICG				(49,490)
Net loss resulting from early retirement and refinancing of debt			(17,349)
Other nonoperating expense, Total			(17,349)	(49,490)
Income (loss) before income taxes	61,709	(2,306)	(643,630)	70,218
Provision for (benefit from) income taxes	15,958	(11,427)	(256,944)	(1,407)
Net income (loss)	45,751	9,121	(386,686)	71,625
Less: Net income attributable to noncontrolling interest		(231)	(268)	(822)
Net income (loss) attributable to Arch Coal, Inc.	45,751	8,890	(386,954)	70,803
Total comprehensive income (loss)	45,494	6,119	(385,185)	72,511
Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues	1,087,618	1,198,673	3,190,807	3,057,139
Costs, expenses and other
Cost of sales	922,455	971,319	2,696,114	2,382,406
Depreciation, depletion and amortization	125,429	138,827	395,688	318,269
Amortization of acquired sales contracts, net	(4,093)	(12,698)	(22,561)	(5,492)
Legal contingencies	(79,532)		(79,532)
Mine closure and asset impairment costs	(2,194)		523,568	7,316
Goodwill impairment			115,791
Selling, general and administrative expenses	11,353	13,752	36,388	36,441
Change in fair value of coal derivatives and coal trading activities, net	5,840	8,360	(29,827)	9,248
Acquisition and transition costs related to ICG
Other operating (income) expense, net	(44,137)	(25,314)	(105,753)	(74,928)
Total operating expenses	935,121	1,094,246	3,529,876	2,673,260
Income from investment in subsidiaries
Income (loss) from operations	152,497	104,427	(339,069)	383,879
Interest expense, net:
Interest expense	(7,021)	(11,060)	(27,815)	(34,858)
Interest income	14,584	14,023	42,557	40,576
Interest expense, net	7,563	2,963	14,742	5,718
Other non-operating expense
Bridge financing costs related to ICG
Net loss resulting from early retirement and refinancing of debt		(1,708)	(1,693)	(1,958)
Other nonoperating expense, Total		(1,708)	(1,693)	(1,958)
Income (loss) before income taxes	160,060	105,682	(326,020)	387,639
Provision for (benefit from) income taxes
Net income (loss)	160,060	105,682	(326,020)	387,639
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	160,060	105,682	(326,020)	387,639
Total comprehensive income (loss)	158,864	101,255	(324,785)	379,390
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales
Depreciation, depletion and amortization	9		24
Amortization of acquired sales contracts, net
Legal contingencies
Mine closure and asset impairment costs
Goodwill impairment
Selling, general and administrative expenses	1,783	1,058	5,871	2,439
Change in fair value of coal derivatives and coal trading activities, net
Acquisition and transition costs related to ICG
Other operating (income) expense, net	(89)	61	(12,449)	102
Total operating expenses	1,703	1,119	(6,554)	2,541
Income from investment in subsidiaries
Income (loss) from operations	(1,703)	(1,119)	6,554	(2,541)
Interest expense, net:
Interest expense	(649)	(541)	(2,382)	(1,535)
Interest income	1,946	1,176	5,811	4,690
Interest expense, net	1,297	635	3,429	3,155
Other non-operating expense
Bridge financing costs related to ICG
Net loss resulting from early retirement and refinancing of debt
Other nonoperating expense, Total
Income (loss) before income taxes	(406)	(484)	9,983	614
Provision for (benefit from) income taxes			1,581
Net income (loss)	(406)	(484)	8,402	614
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	(406)	(484)	8,402	614
Total comprehensive income (loss)	(406)	(484)	8,402	614
Eliminations [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales	(28,080)	(25,908)	(74,866)	(75,940)
Depreciation, depletion and amortization
Amortization of acquired sales contracts, net
Legal contingencies
Mine closure and asset impairment costs
Goodwill impairment
Selling, general and administrative expenses	(1,920)	(1,796)	(5,422)	(5,328)
Change in fair value of coal derivatives and coal trading activities, net
Acquisition and transition costs related to ICG
Other operating (income) expense, net	30,001	27,704	80,289	81,268
Total operating expenses	1		1
Income from investment in subsidiaries	(159,653)	(105,198)	316,041	(388,253)
Income (loss) from operations	(159,654)	(105,198)	316,040	(388,253)
Interest expense, net:
Interest expense	23,549	17,951	64,415	54,569
Interest income	(23,549)	(17,951)	(64,415)	(54,569)
Interest expense, net
Other non-operating expense
Bridge financing costs related to ICG
Net loss resulting from early retirement and refinancing of debt
Other nonoperating expense, Total
Income (loss) before income taxes	(159,654)	(105,198)	316,040	(388,253)
Provision for (benefit from) income taxes
Net income (loss)	(159,654)	(105,198)	316,040	(388,253)
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	(159,654)	(105,198)	316,040	(388,253)
Total comprehensive income (loss)	$ (159,653)	$ (105,198)	$ 316,044	$ (388,253)