Consolidated Statements Of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Statement [Abstract]
Revenues	$ 4,159,038	$ 4,285,895	$ 3,186,268
Costs, expenses and other
Cost of sales	3,438,013	3,267,910	2,395,812
Depreciation, depletion and amortization	525,508	466,587	365,066
Amortization of acquired sales contracts, net	(25,189)	(22,069)	35,606
Change in fair value of coal derivatives and coal trading activities, net	(16,590)	(2,907)	8,924
Coal derivative settlements, non-hedging	(43,990)	7	(4,542)
Selling, general and administrative expenses	134,299	119,056	118,177
Contract settlement resulting from Patriot Coal bankruptcy	58,335
Legal contingencies	(79,532)
Mine closure and asset impairment costs	523,568	7,316
Goodwill and other intangible asset impairment	346,423
Acquisition And Transition Costs		47,360
Gain on Knight Hawk transaction			(41,577)
Other operating income, net	(20,219)	(10,941)	(15,182)
Total operating expenses	4,840,626	3,872,319	2,862,284
Income (loss) from operations	(681,588)	413,576	323,984
Interest expense, net:
Interest expense	(317,626)	(230,186)	(142,549)
Interest income	5,478	3,309	2,449
Interest expense, net	(312,148)	(226,877)	(140,100)
Other nonoperating expense:
Net loss resulting from early retirement and refinancing of debt	(23,668)	(1,958)	(6,776)
Bridge financing costs related to ICG		(49,490)
Other nonoperating expense, Total	(23,668)	(51,448)	(6,776)
Income (loss) before income taxes	(1,017,404)	135,251	177,108
Provision for (benefit from) income taxes	(333,717)	(7,589)	17,714
Net income (loss)	(683,687)	142,840	159,394
Less: Net income attributable to noncontrolling interest	(268)	(1,157)	(537)
Net income (loss) attributable to Arch Coal, Inc.	$ (683,955)	$ 141,683	$ 158,857
Earnings per common share
Basic earnings (loss) per common share	$ (3.24)	$ 0.75	$ 0.98
Diluted earnings (loss) per common share	$ (3.24)	$ 0.74	$ 0.97
Weighted average shares outstanding
Basic, weighted average shares outstanding	211,381	190,086	162,398
Diluted, weighted average shares outstanding	211,381	190,905	163,210
Dividends declared per common share	$ 0.20	$ 0.43	$ 0.39