Condensed Consolidated Statements Of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Condensed Consolidated Statements Of Income [Abstract]
Revenues	$ 1,039,651	$ 872,938
Costs, expenses and other
Cost of sales	850,871	653,684
Depreciation, depletion and amortization	139,966	83,537
Amortization of acquired sales contracts, net	(14,017)	5,944
Selling, general and administrative expenses	30,861	30,435
Change in fair value of coal derivatives and coal trading activities, net	(3,613)	(1,784)
Other operating income, net	(18,498)	(1,116)
Total operating expenses	985,570	770,700
Income from operations	54,081	102,238
Interest expense, net:
Interest expense	(74,772)	(34,580)
Interest income	1,021	746
Interest expense, net	(73,751)	(33,834)
Income before income taxes	(19,670)	68,404
Provision for (benefit from) income taxes	(21,079)	12,530
Net income	1,409	55,874
Less: Net income attributable to noncontrolling interest	(203)	(273)
Net income attributable to Arch Coal, Inc.	$ 1,206	$ 55,601
Earnings per common share
Basic earnings per common share	$ 0.01	$ 0.34
Diluted earnings per common share	$ 0.01	$ 0.34
Basic, weighted average shares outstanding	211,687	162,576
Diluted, weighted average shares outstanding	211,908	163,773
Dividends declared per common share	$ 0.11	$ 0.10