Supplemental Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Assets        
Cash and cash equivalents $ 689,972 $ 734,231 $ 865,761 $ 911,099
Short term investments 249,518 248,954    
Receivables 213,749 232,017    
Inventories 240,113 190,253    
Other 137,637 137,296    
Total current assets 1,530,989 1,542,751    
Property, plant and equipment, net 6,371,335 6,453,458    
Other 412,616 433,514    
Total other assets 412,616 433,514    
Total assets 8,314,940 8,429,723    
Liabilities and Stockholders’ Equity        
Accounts payable 167,480 180,113    
Accrued expenses and other current liabilities 321,503 302,396    
Current maturities of debt 34,368 36,885    
Total current liabilities 523,351 519,394    
Long-term debt 5,117,982 5,123,485    
Asset retirement obligations 404,844 398,896    
Accrued pension benefits 14,436 16,260    
Accrued postretirement benefits other than pension 34,453 32,668    
Accrued workers’ compensation 98,683 94,291    
Deferred income taxes 419,064 422,809    
Other noncurrent liabilities 141,804 153,766    
Total liabilities 6,754,617 6,761,569    
Stockholders' equity 1,560,323 1,668,154    
Total liabilities and stockholders’ equity 8,314,940 8,429,723    
Parent/Issuer [Member]        
Assets        
Cash and cash equivalents 577,832 572,185 753,902 799,333
Short term investments 249,518 248,954    
Receivables 10,184 9,656    
Inventories 0 0    
Other 82,714 89,211    
Total current assets 920,248 920,006    
Property, plant and equipment, net 9,860 10,470    
Investment in subsidiaries 7,487,172 7,464,221    
Intercompany receivables 0 0    
Note receivable from Arch Western 675,000 675,000    
Other 125,994 131,884    
Total other assets 8,288,166 8,271,105    
Total assets 9,218,274 9,201,581    
Liabilities and Stockholders’ Equity        
Accounts payable 12,377 23,394    
Accrued expenses and other current liabilities 117,096 85,899    
Current maturities of debt 24,645 27,625    
Total current liabilities 154,118 136,918    
Long-term debt 5,081,468 5,084,839    
Intercompany payables 1,946,138 1,817,755    
Note payable to Arch Coal 0      
Asset retirement obligations 1,000 981    
Accrued pension benefits 5,323 5,967    
Accrued postretirement benefits other than pension 4,643 4,430    
Accrued workers’ compensation 10,046 9,172    
Deferred income taxes 419,064 422,809    
Other noncurrent liabilities 36,530 50,919    
Total liabilities 7,658,330 7,533,790    
Stockholders' equity 1,559,944 1,667,791    
Total liabilities and stockholders’ equity 9,218,274 9,201,581    
Guarantor Subsidiaries [Member]        
Assets        
Cash and cash equivalents 100,430 150,358 100,353 100,418
Short term investments 0 0    
Receivables 10,894 15,933    
Inventories 240,113 190,253    
Other 44,828 41,455    
Total current assets 396,265 397,999    
Property, plant and equipment, net 6,361,096 6,442,623    
Investment in subsidiaries 0 0    
Intercompany receivables 2,139,722 2,021,110    
Note receivable from Arch Western 0 0    
Other 285,227 300,058    
Total other assets 2,424,949 2,321,168    
Total assets 9,182,310 9,161,790    
Liabilities and Stockholders’ Equity        
Accounts payable 155,080 156,664    
Accrued expenses and other current liabilities 208,275 220,017    
Current maturities of debt 9,723 9,260    
Total current liabilities 373,078 385,941    
Long-term debt 36,514 38,646    
Intercompany payables   0    
Note payable to Arch Coal 675,000 675,000    
Asset retirement obligations 403,844 397,915    
Accrued pension benefits 9,113 10,293    
Accrued postretirement benefits other than pension 29,810 28,238    
Accrued workers’ compensation 88,637 85,119    
Deferred income taxes   0    
Other noncurrent liabilities 105,069 102,461    
Total liabilities 1,721,065 1,723,613    
Stockholders' equity 7,461,245 7,438,177    
Total liabilities and stockholders’ equity 9,182,310 9,161,790    
Non-Guarantor Subsidiaries [Member]        
Assets        
Cash and cash equivalents 11,710 11,688 11,506 11,348
Short term investments 0 0    
Receivables 197,321 211,043    
Inventories 0 0    
Other 10,095 6,630    
Total current assets 219,126 229,361    
Property, plant and equipment, net   2    
Investment in subsidiaries 0 0    
Intercompany receivables 0 0    
Note receivable from Arch Western 0 0    
Other 1,395 1,572    
Total other assets 1,395 1,572    
Total assets 220,521 230,935    
Liabilities and Stockholders’ Equity        
Accounts payable 23 55    
Accrued expenses and other current liabilities 782 1,095    
Current maturities of debt 0 0    
Total current liabilities 805 1,150    
Long-term debt 0 0    
Intercompany payables 193,584 203,355    
Note payable to Arch Coal 0 0    
Asset retirement obligations 0 0    
Accrued pension benefits 0 0    
Accrued postretirement benefits other than pension 0 0    
Accrued workers’ compensation 0 0    
Deferred income taxes 0 0    
Other noncurrent liabilities 205 386    
Total liabilities 194,594 204,891    
Stockholders' equity 25,927 26,044    
Total liabilities and stockholders’ equity 220,521 230,935    
Eliminations [Member]        
Assets        
Cash and cash equivalents 0 0 0 0
Short term investments 0 0    
Receivables (4,650) (4,615)    
Inventories 0 0    
Other 0      
Total current assets (4,650) (4,615)    
Property, plant and equipment, net 379 363    
Investment in subsidiaries (7,487,172) (7,464,221)    
Intercompany receivables (2,139,722) (2,021,110)    
Note receivable from Arch Western (675,000) (675,000)    
Other   0    
Total other assets (10,301,894) (10,160,331)    
Total assets (10,306,165) (10,164,583)    
Liabilities and Stockholders’ Equity        
Accounts payable 0 0    
Accrued expenses and other current liabilities (4,650) (4,615)    
Current maturities of debt 0 0    
Total current liabilities (4,650) (4,615)    
Long-term debt 0 0    
Intercompany payables (2,139,722) (2,021,110)    
Note payable to Arch Coal (675,000) (675,000)    
Asset retirement obligations 0 0    
Accrued pension benefits 0 0    
Accrued postretirement benefits other than pension 0 0    
Accrued workers’ compensation 0 0    
Deferred income taxes 0 0    
Other noncurrent liabilities 0 0    
Total liabilities (2,819,372) (2,700,725)    
Stockholders' equity (7,486,793) (7,463,858)    
Total liabilities and stockholders’ equity $ (10,306,165) $ (10,164,583)