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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Condensed Financial Statement Line Items
Revenues	$ 791,269	$ 975,170	$ 2,294,971	$ 2,901,092
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	688,712	808,489	1,994,653	2,414,306
Depreciation, depletion and amortization	106,323	118,942	327,601	374,631
Amortization of acquired sales contracts, net	(2,568)	(4,093)	(7,587)	(22,561)
Change in fair value of coal derivatives and coal trading activities, net	9,753	5,840	2,053	(29,827)
Asset impairment and mine closure costs	200,397	(2,144)	220,879	523,439
Legal contingencies		(79,532)		(79,532)
Goodwill impairment				115,791
Selling, general and administrative expenses	28,800	33,266	96,311	99,305
Other operating income, net	(5,395)	(24,840)	(16,476)	(44,606)
Operating Expenses	1,026,022	855,928	2,617,434	3,350,946
Loss from investment in subsidiaries
Income (loss) from operations	(234,753)	119,242	(322,463)	(449,854)
Interest expense, net
Interest expense	(95,624)	(75,710)	(285,454)	(229,210)
Interest and investment income	697	1,459	4,749	3,568
Interest expense, net	(94,927)	(74,251)	(280,705)	(225,642)
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt				(19,042)
Income (loss) before income taxes	(329,680)	44,991	(603,168)	(694,538)
Provision for (benefit from) income taxes	(121,913)	20,318	(230,734)	(262,656)
Income (Loss) from Continuing Operations	(207,767)	24,673	(372,434)	(431,882)
Income from discontinued operations, including gain on sale - net of tax	79,404	21,078	101,816	43,618
Net income (loss)	(128,363)	45,751	(270,618)	(388,264)
Less: Net income attributable to noncontrolling interest				(268)
Net income (loss) attributable to Arch Coal, Inc.	(128,363)	45,751	(270,618)	(388,532)
Total comprehensive income (loss)	(125,996)	44,299	(262,717)	(385,523)
Parent/Issuer [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	169	2,437	4,881	7,694
Depreciation, depletion and amortization	1,549	1,399	4,430	3,959
Amortization of acquired sales contracts, net
Change in fair value of coal derivatives and coal trading activities, net
Asset impairment and mine closure costs	57,668		78,150
Legal contingencies
Goodwill impairment
Selling, general and administrative expenses	19,104	22,051	63,006	62,469
Other operating income, net	2,345	(11,052)	(4,663)	(7,693)
Operating Expenses	80,835	14,835	145,804	66,429
Loss from investment in subsidiaries	(61,537)	164,152	(42,199)	(324,429)
Income (loss) from operations	(142,372)	149,317	(188,003)	(390,858)
Interest expense, net
Interest expense	(114,536)	(91,577)	(336,310)	(263,381)
Interest and investment income	6,606	8,329	22,917	19,087
Interest expense, net	(107,930)	(83,248)	(313,393)	(244,294)
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt				(17,349)
Income (loss) before income taxes	(250,302)	66,069	(501,396)	(652,501)
Provision for (benefit from) income taxes	(121,939)	20,318	(230,778)	(264,237)
Income (Loss) from Continuing Operations	(128,363)	45,751	(270,618)	(388,264)
Income from discontinued operations, including gain on sale - net of tax
Net income (loss)				(388,264)
Less: Net income attributable to noncontrolling interest				(268)
Net income (loss) attributable to Arch Coal, Inc.	(128,363)	45,751	(270,618)	(388,532)
Total comprehensive income (loss)	(125,996)	44,299	(262,717)	(385,523)
Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues	791,269	975,170	2,294,971	2,901,092
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)	688,543	806,052	1,989,772	2,406,612
Depreciation, depletion and amortization	104,765	117,534	323,144	370,648
Amortization of acquired sales contracts, net	(2,568)	(4,093)	(7,587)	(22,561)
Change in fair value of coal derivatives and coal trading activities, net	9,753	5,840	2,053	(29,827)
Asset impairment and mine closure costs	142,729	(2,144)	142,729	523,439
Legal contingencies		(79,532)		(79,532)
Goodwill impairment				115,791
Selling, general and administrative expenses	8,214	9,432	28,978	30,965
Other operating income, net	(7,740)	(13,699)	(10,704)	(23,811)
Operating Expenses	943,696	839,390	2,468,385	3,291,724
Loss from investment in subsidiaries
Income (loss) from operations	(152,427)	135,780	(173,414)	(390,632)
Interest expense, net
Interest expense	(6,222)	(6,870)	(18,502)	(27,285)
Interest and investment income	18,975	14,570	49,986	42,508
Interest expense, net	12,753	7,700	31,484	15,223
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt				(1,693)
Income (loss) before income taxes	(139,674)	143,480	(141,930)	(377,102)
Provision for (benefit from) income taxes
Income (Loss) from Continuing Operations	(139,674)	143,480	(141,930)	(377,102)
Income from discontinued operations, including gain on sale - net of tax	79,404	21,078	101,816	43,618
Net income (loss)				(333,484)
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	(60,270)	164,558	(40,114)	(333,484)
Total comprehensive income (loss)	(59,241)	163,375	(39,175)	(336,648)
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	9	27	24
Amortization of acquired sales contracts, net
Change in fair value of coal derivatives and coal trading activities, net
Asset impairment and mine closure costs
Legal contingencies
Goodwill impairment
Selling, general and administrative expenses	1,482	1,783	4,327	5,871
Other operating income, net		(89)	(1,109)	(13,102)
Operating Expenses	1,491	1,703	3,245	(7,207)
Loss from investment in subsidiaries
Income (loss) from operations	(1,491)	(1,703)	(3,245)	7,207
Interest expense, net
Interest expense	(1,070)	(649)	(3,170)	(2,382)
Interest and investment income	1,320	1,946	4,374	5,811
Interest expense, net	250	1,297	1,204	3,429
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt
Income (loss) before income taxes	(1,241)	(406)	(2,041)	10,636
Provision for (benefit from) income taxes	26		44	1,581
Income (Loss) from Continuing Operations	(1,267)	(406)	(2,085)	9,055
Income from discontinued operations, including gain on sale - net of tax
Net income (loss)				9,055
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	(1,267)	(406)	(2,085)	9,055
Total comprehensive income (loss)	(1,267)	(406)	(2,085)	9,055
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
Change in fair value of coal derivatives and coal trading activities, net
Asset impairment and mine closure costs
Legal contingencies
Goodwill impairment
Selling, general and administrative expenses
Other operating income, net
Operating Expenses
Loss from investment in subsidiaries	61,537	(164,152)	42,199	324,429
Income (loss) from operations	61,537	(164,152)	42,199	324,429
Interest expense, net
Interest expense	26,204	23,386	72,528	63,838
Interest and investment income	(26,204)	(23,386)	(72,528)	(63,838)
Interest expense, net
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt
Income (loss) before income taxes	61,537	(164,152)	42,199	324,429
Provision for (benefit from) income taxes
Income (Loss) from Continuing Operations	61,537	(164,152)	42,199	324,429
Income from discontinued operations, including gain on sale - net of tax
Net income (loss)				324,429
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.	61,537	(164,152)	42,199	324,429
Total comprehensive income (loss)	$ 60,508	$ (162,969)	$ 41,260	$ 327,593