Consolidated Statements of Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2009	$ 2,115,106	$ 1,643	$ 1,721,230	$ (53,848)	$ 465,934	$ (19,853)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income attributable to Arch Coal, Inc.	158,857				158,857
Pension, postretirement and other post-employment benefits	9,814
Net amount reclassified to income	101
Unrealized losses on available-for- sale securities	1,841
Unrealized gains on derivatives	221
Net amount reclassified to income	1,514
Total comprehensive income (loss)	172,293					13,436
Dividends, Common Stock	(63,373)				(63,373)
Issuance of shares of common stock under the stock incentive plan — restricted stock and restricted stock units, net of forfeitures	0	0	0
Issuance of shares of common stock under the stock incentive plan — stock options including income tax benefits	1,764	2	1,762
Employee stock-based compensation expense	11,717		11,717
Share-based Compensation	11,717
Ending Balance at Dec. 31, 2010	2,237,507	1,645	1,734,709	(53,848)	561,418	(6,417)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income attributable to Arch Coal, Inc.	141,683				141,683
Pension, postretirement and other post-employment benefits	4,271
Net amount reclassified to income	1,665
Unrealized losses on available-for- sale securities	114
Unrealized gains on derivatives	2,913
Net amount reclassified to income	(10,563)
Total comprehensive income (loss)	140,150					(1,533)
Dividends, Common Stock	(80,748)				(80,748)
Issuance of shares of common stock under the stock incentive plan — restricted stock and restricted stock units, net of forfeitures	0	2	(2)
Issuance of shares of common stock under the stock incentive plan — stock options including income tax benefits	2,316	2	2,314
Employee stock-based compensation expense	10,882		10,882
Share-based Compensation	10,882
Stock Issued During Period, Value, New Issues	1,267,933	487	1,267,446
Ending Balance at Dec. 31, 2011	3,578,040	2,136	3,015,349	(53,848)	622,353	(7,950)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income attributable to Arch Coal, Inc.	(683,955)				(683,955)
Pension, postretirement and other post-employment benefits	(14,523)
Net amount reclassified to income	918
Unrealized losses on available-for- sale securities	(1,924)
Unrealized gains on derivatives	4,320
Net amount reclassified to income	2,653
Total comprehensive income (loss)	(692,512)					(8,557)
Dividends, Common Stock	(42,440)				(42,440)
Noncontrolling Interest, Change in Redemption Value	(5,474)		(5,474)
Issuance of shares of common stock under the stock incentive plan — restricted stock and restricted stock units, net of forfeitures	0	0	0
Issuance of shares of common stock under the stock incentive plan — stock options including income tax benefits	5,131	5	5,126
Employee stock-based compensation expense	11,822
Share-based Compensation	11,822		11,822
Ending Balance at Dec. 31, 2012	$ 2,854,567	$ 2,141	$ 3,026,823	$ (53,848)	$ (104,042)	$ (16,507)