Consolidated Statements Of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Revenues	$ 791,269	$ 975,170	$ 2,294,971	$ 2,901,092
Costs and Expenses [Abstract]
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
Goodwill impairment				115,791
Legal contingencies		(79,532)		(79,532)
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)
Total operating expenses	1,026,022	855,928	2,617,434	3,350,946
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 expense:
Net loss resulting from early retirement and refinancing of debt				(19,042)
Loss from continuing operations 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)
Earnings (loss) per common share
Income (Loss) from Continuing Operations, Per Basic Share	$ (0.98)	$ 0.12	$ (1.76)	$ (2.04)
Income (Loss) from Continuing Operations, Per Diluted Share	$ (0.98)	$ 0.12	$ (1.76)	$ (2.04)
Basic earnings (loss) per common share	$ (0.61)	$ 0.22	$ (1.28)	$ (1.83)
Diluted earnings (loss) per common share	$ (0.61)	$ 0.22	$ (1.28)	$ (1.83)
Weighted average shares outstanding
Basic, weighted average shares outstanding	212,111	212,053	212,085	211,931
Diluted, weighted average shares outstanding	212,111	212,076	212,085	211,931
Dividends declared per common share	$ 0.03	$ 0.03	$ 0.09	$ 0.17