Consolidated Statements Of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Consolidated Statements Of Income [Abstract]
REVENUES	$ 1,228,756	$ 1,198,673	$ 985,528	$ 872,938	$ 835,394	$ 874,705	$ 764,295	$ 711,874	$ 4,285,895	$ 3,186,268	$ 2,576,081
COSTS, EXPENSES AND OTHER
Cost of sales									3,267,910	2,395,812	2,070,715
Depreciation, depletion and amortization									466,587	365,066	301,608
Amortization of acquired sales contracts, net									(22,069)	35,606	19,623
Selling, general and administrative expenses									119,056	118,177	97,787
Change in fair value of coal derivatives and coal trading activities, net									(2,907)	8,924	(12,056)
Acquisition and transition costs									54,676
Gain on Knight Hawk transaction							(41,600)			(41,577)
Other operating income, net									(10,934)	(19,724)	(39,036)
Total operating expenses									3,872,319	2,862,284	2,452,367
Income from operations	139,728	76,256	95,354	102,238	86,938	98,347	106,499	32,200	413,576	323,984	123,714
Interest expense, net:
Interest expense									(230,186)	(142,549)	(105,932)
Interest income									3,309	2,449	7,622
Interest expense, net									(226,877)	(140,100)	(98,310)
Other non-operating expense:
Bridge financing costs related to ICG									(49,490)
Net loss resulting from early retirement of debt									(1,958)	(6,776)
Other non-operating expense, Total									(51,448)	(6,776)
Income before income taxes									135,251	177,108	25,404
Provision for (benefit from) income taxes									(7,589)	17,714	(16,775)
Net income	71,215	9,121	6,630	55,874	48,031	46,859	66,274	(1,770)	142,840	159,394	42,179
Less: Net income attributable to noncontrolling interest									(1,157)	(537)	(10)
Net income attributable to Arch Coal, Inc.									$ 141,683	$ 158,857	$ 42,169
EARNINGS PER COMMON SHARE
Basic earnings per common share	$ 0.34	$ 0.04	$ 0.04	$ 0.34	$ 0.29	$ 0.29	$ 0.41	$ (0.01)	$ 0.75	$ 0.98	$ 0.28
Diluted earnings per common share	$ 0.33	$ 0.04	$ 0.04	$ 0.34	$ 0.29	$ 0.29	$ 0.41	$ (0.01)	$ 0.74	$ 0.97	$ 0.28
Basic weighted average shares outstanding									190,086	162,398	150,963
Diluted weighted average shares outstanding									190,905	163,210	151,272
Dividends declared per common share									$ 0.43	$ 0.39	$ 0.36