Condensed Consolidated Statements Of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
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)
Change in fair value of coal derivatives and coal trading activities, net	5,840	8,360	(29,827)	9,248
Selling, general and administrative expenses	33,266	33,275	99,305	92,749
Legal contingencies	(79,532)		(79,532)
Mine closure and asset impairment costs	(2,194)		523,568	7,316
Goodwill impairment			115,791
Acquisition and transition costs related to ICG		4,694		46,044
Other operating income, net	(25,276)	(3,611)	(45,605)	(9,018)
Total operating expenses	951,658	1,122,417	3,589,750	2,783,291
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 nonoperating 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
Earnings per common share
Basic earnings (loss) per common share	$ 0.22	$ 0.04	$ (1.83)	$ 0.39
Diluted earnings (loss) per common share	$ 0.22	$ 0.04	$ (1.83)	$ 0.39
Weighted average shares outstanding
Basic, weighted average shares outstanding	212,053	211,337	211,931	182,898
Diluted, weighted average shares outstanding	212,076	211,974	211,931	183,850
Dividends declared per common share	$ 0.03	$ 0.11	$ 0.17	$ 0.32