Condensed Consolidated Statements Of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Condensed Consolidated Statements Of Operations [Abstract]
Revenues	$ 1,063,538	$ 985,528	$ 2,103,189	$ 1,858,466
Costs, expenses and other
Cost of sales	881,259	715,590	1,732,130	1,369,274
Depreciation, depletion and amortization	132,868	97,236	272,834	180,773
Amortization of acquired sales contracts, net	(4,451)	1,262	(18,468)	7,206
Mine closure and asset impairment costs	525,762		525,762
Goodwill impairment	115,791		115,791
Selling, general and administrative expenses	35,178	29,040	66,039	59,474
Change in fair value of coal derivatives and coal trading activities, net	(32,054)	2,672	(35,667)	888
Acquisition and transition costs related to ICG		48,666		48,666
Other operating income, net	(1,831)	(4,292)	(20,329)	(5,407)
Total operating expenses	1,652,522	890,174	2,638,092	1,660,874
Income (loss) from operations	(588,984)	95,354	(534,903)	197,592
Interest expense, net:
Interest expense	(78,728)	(42,249)	(153,500)	(76,829)
Interest income	1,088	755	2,109	1,501
Interest expense, net	(77,640)	(41,494)	(151,391)	(75,328)
Other nonoperating expense:
Bridge financing costs related to ICG		(49,490)		(49,490)
Net loss resulting from early retirement and refinancing of debt	(19,042)	(250)	(19,042)	(250)
Other nonoperating expense, Total	(19,042)	(49,740)	(19,042)	(49,740)
Income (loss) before income taxes	(685,666)	4,120	(705,336)	72,524
Provision for (benefit from) income taxes	(250,242)	(2,510)	(271,321)	10,020
Net income (loss)	(435,424)	6,630	(434,015)	62,504
Less: Net income attributable to noncontrolling interest	(65)	(318)	(268)	(591)
Net income (loss) attributable to Arch Coal, Inc.	$ (435,489)	$ 6,312	$ (434,283)	$ 61,913
Earnings per common share
Basic earnings (loss) per common share	$ (2.05)	$ 0.04	$ (2.05)	$ 0.37
Diluted earnings (loss) per common share	$ (2.05)	$ 0.04	$ (2.05)	$ 0.37
Basic, weighted average shares outstanding	212,048	174,244	211,868	168,442
Diluted, weighted average shares outstanding	212,048	175,272	211,868	169,554
Dividends declared per common share	$ 0.03	$ 0.11	$ 0.14	$ 0.21