Consolidated Statements Of Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Common Stock [Member]	Paid-In Capital [Member]	Treasury Stock, At Cost [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Total
BALANCE at Dec. 31, 2008	$ 1,447	$ 1,381,496	$ (53,848)	$ 478,734	$ (79,096)	$ 1,728,733
Comprehensive income:
Net income attributable to Arch Coal, Inc.				42,169		42,169
Pension, postretirement and other post-employment benefits					12,176	12,176
Net amount reclassified to income					718	718
Unrealized gains (losses) on available-for- sale securities					(86)	(86)
Unrealized gains (losses) on derivatives					2,436	2,436
Net amount reclassified to income					43,999	43,999
Total comprehensive income						101,412
Dividends:
Dividends on common shares ($0.36, 0.39 and 0.43 per share for the years 2009, 2010 and 2011 respectively)						(54,969)
Issuance of common shares (19,550 and 48,705 in 2009 and 2011 respectively)	196	326,256				326,452
Issuance of 45, 9 and 162 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - restricted stock and restricted stock units, net of forfeitures	0	0				0
Issuance of 13, 155 and 199 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - stock options including income tax benefits	0	84				84
Employee stock-based compensation expense		13,394				13,394
BALANCE at Dec. 31, 2009	1,643	1,721,230	(53,848)	465,934	(19,853)	2,115,106
Comprehensive income:
Net income attributable to Arch Coal, Inc.				158,857		158,857
Pension, postretirement and other post-employment benefits					9,750	9,750
Net amount reclassified to income					110	110
Unrealized gains (losses) on available-for- sale securities					1,841	1,841
Unrealized gains (losses) on derivatives					221	221
Net amount reclassified to income					1,514	1,514
Total comprehensive income						172,293
Dividends:
Dividends on common shares ($0.36, 0.39 and 0.43 per share for the years 2009, 2010 and 2011 respectively)				(63,373)		(63,373)
Issuance of 45, 9 and 162 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - restricted stock and restricted stock units, net of forfeitures	0	0				0
Issuance of 13, 155 and 199 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - stock options including income tax benefits	2	1,762				1,764
Employee stock-based compensation expense		11,717				11,717
BALANCE at Dec. 31, 2010	1,645	1,734,709	(53,848)	561,418	(6,417)	2,237,507
Comprehensive income:
Net income attributable to Arch Coal, Inc.				141,683		141,683
Pension, postretirement and other post-employment benefits					4,331	4,331
Net amount reclassified to income					1,672	1,672
Unrealized gains (losses) on available-for- sale securities					114	114
Unrealized gains (losses) on derivatives					2,913	2,913
Net amount reclassified to income					(10,563)	(10,563)
Total comprehensive income						140,150
Dividends:
Dividends on common shares ($0.36, 0.39 and 0.43 per share for the years 2009, 2010 and 2011 respectively)				(80,748)		(80,748)
Issuance of common shares (19,550 and 48,705 in 2009 and 2011 respectively)	487	1,267,446				1,267,933
Issuance of 45, 9 and 162 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - restricted stock and restricted stock units, net of forfeitures	2	(2)				0
Issuance of 13, 155 and 199 shares of common stock for the years 2009, 2010 and 2011 respectively under the stock incentive plan - stock options including income tax benefits	2	2,314				2,316
Employee stock-based compensation expense		10,882				10,882
BALANCE at Dec. 31, 2011	$ 2,136	$ 3,015,349	$ (53,848)	$ 622,353	$ (7,950)	$ 3,578,040