Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Statements Of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statement Line Items
Revenues	$ 968,231	$ 1,087,618	$ 1,063,538	$ 1,039,651	$ 1,228,756	$ 1,198,673	$ 985,528	$ 872,938	$ 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	0	(2,194)	525,762	0					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) expense, net									(20,219)	(10,941)	(15,182)
Total operating expenses									4,840,626	3,872,319	2,862,284
Income from investment in subsidiaries
Income (loss) from operations	(282,645)	135,960	(588,984)	54,081	139,728	76,256	95,354	102,238	(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 non-operating 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)	(295,423)	45,751	(435,424)	1,409	71,215	9,121	6,630	55,874	(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
Total comprehensive income (loss)									(692,239)	141,240	172,885
Parent/Issuer [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales									10,918	22,926	11,523
Depreciation, depletion and amortization									5,392	2,876	2,933
Amortization of acquired sales contracts, net
Change in fair value of coal derivatives and coal trading activities, net
Coal derivative settlements, non-hedging
Selling, general and administrative expenses									84,198	74,589	79,580
Contract settlement resulting from Patriot Coal bankruptcy
Legal contingencies
Mine closure and asset impairment costs										7,316
Goodwill and other intangible asset impairment
Acquisition And Transition Costs										47,360
Gain on Knight Hawk transaction
Other operating (income) expense, net									(13,391)	(23,306)	(10,258)
Total operating expenses									87,117	131,761	83,778
Income from investment in subsidiaries									(569,795)	556,442	393,363
Income (loss) from operations									(656,912)	424,681	309,585
Interest expense, net:
Interest expense									(366,614)	(256,221)	(143,606)
Interest income									28,097	16,281	11,129
Interest expense, net									(338,517)	(239,940)	(132,477)
Other non-operating expense
Net loss resulting from early retirement and refinancing of debt									(21,975)
Bridge financing costs related to ICG										(49,490)
Other nonoperating expense, Total									(21,975)	(49,490)
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
Total comprehensive income (loss)										141,240	172,885
Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues									4,159,038	4,285,895	3,186,268
Costs, expenses and other
Cost of sales									3,427,095	3,244,984	2,384,289
Depreciation, depletion and amortization									520,083	463,711	362,133
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									41,316	40,940	36,091
Contract settlement resulting from Patriot Coal bankruptcy									58,335
Legal contingencies									(79,532)
Mine closure and asset impairment costs									523,568
Goodwill and other intangible asset impairment									346,423
Acquisition And Transition Costs
Gain on Knight Hawk transaction											(41,577)
Other operating (income) expense, net									9,559	12,615	(4,924)
Total operating expenses									4,761,078	3,737,281	2,776,000
Income from investment in subsidiaries
Income (loss) from operations									(602,040)	548,614	410,268
Interest expense, net:
Interest expense									(35,207)	(46,565)	(64,377)
Interest income									57,303	55,072	52,899
Interest expense, net									22,096	8,507	(11,478)
Other non-operating expense
Net loss resulting from early retirement and refinancing of debt									(1,693)	(1,958)	(6,776)
Bridge financing costs related to ICG
Other nonoperating expense, Total									(1,693)	(1,958)	(6,776)
Income (loss) before income taxes									(581,637)	555,163	392,014
Provision for (benefit from) income taxes
Net income (loss)									(581,637)	555,163	392,014
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.									(581,637)	555,163	392,014
Total comprehensive income (loss)										552,663	400,664
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales
Depreciation, depletion and amortization									33
Amortization of acquired sales contracts, net
Change in fair value of coal derivatives and coal trading activities, net
Coal derivative settlements, non-hedging
Selling, general and administrative expenses									8,785	3,527	2,506
Contract settlement resulting from Patriot Coal bankruptcy
Legal contingencies
Mine closure and asset impairment costs
Goodwill and other intangible asset impairment
Acquisition And Transition Costs
Gain on Knight Hawk transaction
Other operating (income) expense, net									(16,387)	(250)
Total operating expenses									(7,569)	3,277	2,506
Income from investment in subsidiaries
Income (loss) from operations									7,569	(3,277)	(2,506)
Interest expense, net:
Interest expense									(3,221)	(2,224)	(2,849)
Interest income									7,494	6,780	6,704
Interest expense, net									4,273	4,556	3,855
Other non-operating expense
Net loss resulting from early retirement and refinancing of debt
Bridge financing costs related to ICG
Other nonoperating expense, Total
Income (loss) before income taxes									11,842	1,279	1,349
Provision for (benefit from) income taxes
Net income (loss)									11,842	1,279	1,349
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.									11,842	1,279	1,349
Total comprehensive income (loss)										1,279	1,349
Eliminations [Member]
Condensed Financial Statement Line Items
Revenues
Costs, expenses and other
Cost of sales
Depreciation, depletion and amortization
Amortization of acquired sales contracts, net
Change in fair value of coal derivatives and coal trading activities, net
Coal derivative settlements, non-hedging
Selling, general and administrative expenses
Contract settlement resulting from Patriot Coal bankruptcy
Legal contingencies
Mine closure and asset impairment costs
Goodwill and other intangible asset impairment
Acquisition And Transition Costs
Gain on Knight Hawk transaction
Other operating (income) expense, net
Total operating expenses
Income from investment in subsidiaries									569,795	(556,442)	(393,363)
Income (loss) from operations									569,795	(556,442)	(393,363)
Interest expense, net:
Interest expense									87,416	74,824	68,283
Interest income									(87,416)	(74,824)	(68,283)
Interest expense, net
Other non-operating expense
Net loss resulting from early retirement and refinancing of debt
Bridge financing costs related to ICG
Other nonoperating expense, Total
Income (loss) before income taxes									569,795	(556,442)	(393,363)
Provision for (benefit from) income taxes
Net income (loss)									569,795	(556,442)	(393,363)
Less: Net income attributable to noncontrolling interest
Net income (loss) attributable to Arch Coal, Inc.									569,795	(556,442)	(393,363)
Total comprehensive income (loss)										$ (553,942)	$ (402,013)