Supplemental Consolidating Financial Information (Schedule of Consolidating Statements of Operations) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Income Statements, Captions [Line Items]
Dividends declared per common share									$ 0.01	$ 0.12	$ 0.20
Revenues	$ 745,192	$ 742,180	$ 713,776	$ 735,971	$ 719,386	$ 791,269	$ 766,332	$ 737,370	$ 2,937,119	$ 3,014,357	$ 3,768,126
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)									2,566,193	2,663,136	3,155,099
Depreciation, depletion and amortization									418,748	426,442	492,211
Amortization of acquired sales contracts, net									(13,187)	(9,457)	(25,189)
Change in fair value of coal derivatives and coal trading activities, net									(3,686)	7,845	(16,590)
Asset impairment and mine closure costs	17,541	5,060	1,512	0	0	200,397	20,482	0	24,113	220,879	539,182
Goodwill impairment						0	0	0	0	265,423	330,680
Contract settlement resulting from Patriot Coal bankruptcy									0	0	58,335
Reduction in accrual related to acquired litigation										0	(79,532)
Selling, general and administrative expenses									114,223	133,448	134,299
Other operating income, net									(19,754)	(30,218)	(63,357)
Total operating expenses									3,086,650	3,677,498	4,525,138
Loss from investment in subsidiaries									0
Loss from operations	(5,303)	(35,300)	(35,805)	(73,123)	(340,678)	(234,753)	(36,279)	(51,431)	(149,531)	(663,141)	(757,012)
Interest expense, net
Interest expense									(390,946)	(381,267)	(317,615)
Interest and investment income									7,758	6,603	5,473
Interest expense, net									(383,188)	(374,664)	(312,142)
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement of debt and financing activities										(42,921)	(23,668)
Net loss resulting from early retirement and refinancing of debt									0	(42,921)	(23,668)
Loss from continuing operations before income taxes									(532,719)	(1,080,726)	(1,092,822)
Provision for (benefit from) income taxes									25,634	(335,498)	(353,907)
Loss from continuing operations					(372,794)	(207,767)	(80,351)	(84,316)	(558,353)	(745,228)	(738,915)
Income from discontinued operations, including gain on sale - net of tax					1,580	79,404	8,145	14,267	0	103,396	55,228
Net income (loss)	(240,135)	(97,218)	(96,860)	(124,140)	(371,214)	(128,363)	(72,206)	(70,049)	(558,353)	(641,832)	(683,687)
Less: Net income attributable to noncontrolling interest										0	(268)
Net loss attributable to Arch Coal, Inc.									(558,353)	(641,832)	(683,955)
Total comprehensive income (loss)									(592,804)	(587,633)	(692,239)
Parent/Issuer [Member]
Condensed Income Statements, Captions [Line Items]
Revenues									0
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)									3,016	9,117	10,921
Depreciation, depletion and amortization									5,154	5,949	5,392
Amortization of acquired sales contracts, net									0
Change in fair value of coal derivatives and coal trading activities, net									0
Asset impairment and mine closure costs									3,642	78,150
Goodwill impairment										0
Reduction in accrual related to acquired litigation											0
Selling, general and administrative expenses									79,902	88,820	84,199
Other operating income, net									(4,480)	4,209	(13,392)
Total operating expenses									87,234	186,245	87,120
Loss from investment in subsidiaries									(13,085)	(328,889)	(589,665)
Loss from operations									(100,319)	(515,134)	(676,785)
Interest expense, net
Interest expense									463,823	(449,614)	(366,584)
Interest and investment income									31,389	30,285	27,750
Interest expense, net									(432,434)	(419,329)	(338,834)
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt										(42,921)	(21,975)
Loss from continuing operations before income taxes									(532,753)	(977,384)	(1,037,594)
Provision for (benefit from) income taxes									25,600	(335,552)	(353,907)
Loss from continuing operations										(641,832)	(683,687)
Income from discontinued operations, including gain on sale - net of tax										0
Net income (loss)											(683,687)
Less: Net income attributable to noncontrolling interest											(268)
Net loss attributable to Arch Coal, Inc.									(558,353)	(641,832)	(683,955)
Total comprehensive income (loss)									(592,804)	(587,633)	(692,239)
Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Revenues									2,937,119	3,014,357	3,768,126
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)									2,566,572	2,657,583	3,144,178
Depreciation, depletion and amortization									413,559	420,458	486,786
Amortization of acquired sales contracts, net									(13,187)	(9,457)	(25,189)
Change in fair value of coal derivatives and coal trading activities, net									(3,686)	7,845	(16,590)
Asset impairment and mine closure costs									20,471	142,729	539,182
Goodwill impairment										265,423	330,680
Contract settlement resulting from Patriot Coal bankruptcy											58,335
Reduction in accrual related to acquired litigation											(79,532)
Selling, general and administrative expenses									29,739	39,825	44,363
Other operating income, net									(15,726)	(34,856)	(39,209)
Total operating expenses									2,997,742	3,489,550	4,443,004
Loss from investment in subsidiaries									0
Loss from operations									(60,623)	(475,193)	(674,878)
Interest expense, net
Interest expense									26,137	(24,747)	(34,849)
Interest and investment income									74,511	68,248	57,268
Interest expense, net									48,374	43,501	22,419
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt										0	(1,693)
Loss from continuing operations before income taxes									(12,249)	(431,692)	(654,152)
Provision for (benefit from) income taxes									0
Loss from continuing operations										(431,692)	(654,152)
Income from discontinued operations, including gain on sale - net of tax										103,396	55,228
Net income (loss)											(598,924)
Net loss attributable to Arch Coal, Inc.									(12,249)	(328,296)	(598,924)
Total comprehensive income (loss)									(34,439)	(304,278)	(604,903)
Non-Guarantor Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Revenues									0
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)									0
Depreciation, depletion and amortization									35	35	33
Amortization of acquired sales contracts, net									0
Change in fair value of coal derivatives and coal trading activities, net									0
Asset impairment and mine closure costs									0
Goodwill impairment										0
Reduction in accrual related to acquired litigation											0
Selling, general and administrative expenses									6,626	7,038	8,785
Other operating income, net									(4,987)	(5,370)	(13,804)
Total operating expenses									1,674	1,703	(4,986)
Loss from investment in subsidiaries									0
Loss from operations									(1,674)	(1,703)	4,986
Interest expense, net
Interest expense									4,259	(4,214)	(3,221)
Interest and investment income									5,131	5,378	7,494
Interest expense, net									872	1,164	4,273
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt										0
Loss from continuing operations before income taxes									(802)	(539)	9,259
Provision for (benefit from) income taxes									34	54
Loss from continuing operations										(593)	9,259
Income from discontinued operations, including gain on sale - net of tax										0
Net income (loss)											9,259
Net loss attributable to Arch Coal, Inc.									(836)	(593)	9,259
Total comprehensive income (loss)									(836)	(593)	9,259
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Revenues									0
Costs, expenses and other
Cost of sales (exclusive of items shown separately below)									(3,395)	(3,564)
Depreciation, depletion and amortization									0
Amortization of acquired sales contracts, net									0
Change in fair value of coal derivatives and coal trading activities, net									0
Asset impairment and mine closure costs									0
Goodwill impairment										0
Reduction in accrual related to acquired litigation											0
Selling, general and administrative expenses									(2,044)	(2,235)	(3,048)
Other operating income, net									5,439	5,799	3,048
Total operating expenses									0
Loss from investment in subsidiaries									13,085	328,889	589,665
Loss from operations									13,085	328,889	589,665
Interest expense, net
Interest expense									(103,273)	97,308	87,039
Interest and investment income									(103,273)	(97,308)	(87,039)
Interest expense, net									0	0	0
Other Nonoperating Income (Expense) [Abstract]
Net loss resulting from early retirement and refinancing of debt										0
Loss from continuing operations before income taxes									13,085	328,889	589,665
Provision for (benefit from) income taxes									0
Loss from continuing operations										328,889	589,665
Income from discontinued operations, including gain on sale - net of tax										0
Net income (loss)											589,665
Net loss attributable to Arch Coal, Inc.									13,085	328,889	589,665
Total comprehensive income (loss)									$ 35,275	$ 304,871	$ 595,644