Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Condensed Financial Statement Line Items
Revenues	$ 825,502	$ 1,039,651
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	710,573	850,871
Depreciation, depletion and amortization	118,868	139,966
Amortization of acquired sales contracts, net	(2,810)	(14,017)
ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet	1,308	(3,613)
Selling, general and administrative expenses	33,209	30,861
Other operating (income) expense, net	(3,217)	(18,498)
Total operating expenses	857,931	985,570
Income from investment in subsidiaries
Income (loss) from operations	(32,429)	54,081
Interest expense, net:
Interest expense	(95,087)	(74,772)
Interest and investment income	2,836	1,021
Interest expense, net	(92,251)	(73,751)
Income (loss) before income taxes	(124,680)	(19,670)
Provision for (benefit from) income taxes	(54,631)	(21,079)
Net income (loss)	(70,049)	1,409
Less: Net income attributable to noncontrolling interest	0	(203)
Net income (loss) attributable to Arch Coal, Inc.	(70,049)	1,206
Total comprehensive income (loss)	(68,558)	8,709
Parent/Issuer [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	2,883	2,967
Depreciation, depletion and amortization	1,406	1,216
Amortization of acquired sales contracts, net
ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet
Selling, general and administrative expenses	21,698	18,644
Other operating (income) expense, net	(5,907)	(3,111)
Total operating expenses	20,080	19,716
Income from investment in subsidiaries	(2,871)	77,315
Income (loss) from operations	(22,951)	57,599
Interest expense, net:
Interest expense	(110,827)	(82,097)
Interest and investment income	9,098	4,828
Interest expense, net	(101,729)	(77,269)
Income (loss) before income taxes	(124,680)	(19,670)
Provision for (benefit from) income taxes	(54,631)	(22,660)
Net income (loss)	(70,049)	2,990
Less: Net income attributable to noncontrolling interest		(203)
Net income (loss) attributable to Arch Coal, Inc.		2,787
Total comprehensive income (loss)	(68,558)	8,709
Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues	825,502	1,039,651
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	707,690	847,904
Depreciation, depletion and amortization	117,453	138,744
Amortization of acquired sales contracts, net	(2,810)	(14,017)
ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet	1,308	(3,613)
Selling, general and administrative expenses	10,035	9,637
Other operating (income) expense, net	3,702	(3,027)
Total operating expenses	837,378	975,628
Income from investment in subsidiaries
Income (loss) from operations	(11,876)	64,023
Interest expense, net:
Interest expense	(6,455)	(11,484)
Interest and investment income	15,443	14,128
Interest expense, net	8,988	2,644
Income (loss) before income taxes	(2,888)	66,667
Provision for (benefit from) income taxes
Net income (loss)	(2,888)	66,667
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.		66,667
Total comprehensive income (loss)	(3,324)	68,111
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)
Depreciation, depletion and amortization	9	6
Amortization of acquired sales contracts, net
ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet
Selling, general and administrative expenses	1,476	2,580
Other operating (income) expense, net	(1,012)	(12,360)
Total operating expenses	473	(9,774)
Income from investment in subsidiaries
Income (loss) from operations	(473)	9,774
Interest expense, net:
Interest expense	(1,041)	(1,038)
Interest and investment income	1,531	1,912
Interest expense, net	490	874
Income (loss) before income taxes	17	10,648
Provision for (benefit from) income taxes		1,581
Net income (loss)	17	9,067
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.		9,067
Total comprehensive income (loss)	17	9,067
Eliminations [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)
Depreciation, depletion and amortization
Amortization of acquired sales contracts, net
ChangeInFairValueOfCoalDerivativesAndCoalTradingActivitiesNet
Selling, general and administrative expenses
Other operating (income) expense, net
Total operating expenses
Income from investment in subsidiaries	2,871	(77,315)
Income (loss) from operations	2,871	(77,315)
Interest expense, net:
Interest expense	23,236	19,847
Interest and investment income	(23,236)	(19,847)
Interest expense, net
Income (loss) before income taxes	2,871	(77,315)
Provision for (benefit from) income taxes
Net income (loss)	2,871	(77,315)
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.		(77,315)
Total comprehensive income (loss)	$ 3,307	$ (77,178)