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Debt And Financing Arrangements (Narrative) (Details) (USD $)
12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended 0 Months Ended 0 Months Ended 0 Months Ended 12 Months Ended 0 Months Ended 12 Months Ended 12 Months Ended 0 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Jun. 15, 2011
Dec. 31, 2011
7.00% Senior Notes Due June 15, 2019 [Member]
Dec. 31, 2011
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
2019 and 2021 Notes [Member]
Dec. 31, 2011
Revolving Credit Facility [Member]
Dec. 31, 2011
Accounts Receivable Securitization Program [Member]
Dec. 12, 2011
Accounts Receivable Securitization Program [Member]
Jul. 14, 2011
9.125% Senior Notes Due April 1, 2018 [Member]
Jun. 14, 2011
9.125% Senior Notes Due April 1, 2018 [Member]
Jun. 15, 2011
4.00% Convertible Senior Notes Due April 1, 2017 [Member]
Jun. 14, 2011
4.00% Convertible Senior Notes Due April 1, 2017 [Member]
Jun. 15, 2011
9.00% Convertible Senior Notes Due August 1, 2012 [Member]
Jun. 14, 2011
9.00% Convertible Senior Notes Due August 1, 2012 [Member]
Dec. 31, 2011
ICG Debt [Member]
Sep. 08, 2010
6.75% Senior Notes Due July 1, 2013 [Member]
Dec. 31, 2011
6.75% Senior Notes Due July 1, 2013 [Member]
Dec. 31, 2010
6.75% Senior Notes Due July 1, 2013 [Member]
Dec. 31, 2011
8.75% Senior Notes Due August 1, 2016 [Member]
Dec. 31, 2011
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
6.75% Senior Notes [Member]
Dec. 31, 2012
Secured Credit Facility [Member]
years
Dec. 31, 2011
Secured Credit Facility [Member]
Dec. 31, 2010
Secured Credit Facility [Member]
Dec. 31, 2011
Prior to June 15, 2014 [Member]
7.00% Senior Notes Due June 15, 2019 [Member]
Dec. 31, 2011
Prior to June 15, 2014 [Member]
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
Prior to June 15, 2014 [Member]
2019 and 2021 Notes [Member]
Dec. 31, 2011
June 15, 2015 Through June 14, 2016 [Member]
7.00% Senior Notes Due June 15, 2019 [Member]
Dec. 31, 2011
June 15, 2016 Through June 14, 2017 [Member]
7.00% Senior Notes Due June 15, 2019 [Member]
Dec. 31, 2011
June 15, 2016 Through June 14, 2017 [Member]
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
June 15, 2017 And Beyond [Member]
7.00% Senior Notes Due June 15, 2019 [Member]
Dec. 31, 2011
June 15, 2019 and Beyond [Member]
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
June 15, 2017 Through June 14, 2018 [Member]
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
June 15, 2018 Through June 14, 2019 [Member]
7.25% Senior Notes Due 2021 [Member]
Dec. 31, 2011
Redeemable Between August 1, 2013 And July 31, 2014 [Member]
8.75% Senior Notes Due August 1, 2016 [Member]
Dec. 31, 2011
Redeemable Between August 1, 2014 And July 31, 2015 [Member]
8.75% Senior Notes Due August 1, 2016 [Member]
Dec. 31, 2011
August 1, 2015 And Beyond [Member]
Dec. 31, 2011
Redeemable Between October 1, 2015 And September 30, 2016 [Member]
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
Redeemable Between October 1, 2016 And September 30, 2017 [Member]
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
Redeemable Between October 1, 2017 And September 30, 2018 [Member]
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
Redeemable On Or After October 1, 2018 [Member]
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
Redeemable Prior to October 1, 2013 [Member]
7.25% Senior Notes Due October 10, 2020 [Member]
Dec. 31, 2011
Redeemable Prior To August 1, 2012 [Member]
8.75% Senior Notes Due August 1, 2016 [Member]
Jul. 14, 2011
Repayment of Debt [Member]
9.125% Senior Notes Due April 1, 2018 [Member]
Debt Instrument [Line Items]                                                                                            
Debt instrument, interest rate, stated percentage         7.00% 7.25%           9.125%   4.00%   9.00%     6.75%   8.75% 7.25% 6.75%                                              
Debt Instrument, Interest Rate at Period End                 0.73%                                                                          
Restricted cash $ 10,322,000                     $ 260,700,000                                                                    
Subsequent event, senior notes redeemed                                                                                           251,400,000
Accrued interest 213,697,000 134,866,000 76,801,000               5,200,000                                                                      
Debt instrument, convertible, latest date                                 August 17, 2011                                                          
Debt assumed in acquisition       604,800,000                   298,500,000   1,700,000 54,000,000                                                          
Calculated conversion rate                         2,614.6848   2,392.73414                                                              
Principal amount increment for conversion                         1,000   1,000                                                              
Net loss resulting from early retirement of debt 1,958,000 6,776,000                             2,000,000     6,800,000                                                    
Maximum borrowing capacity                 250,000,000 175,000,000                           2,000,000,000                                            
Line of credit facility, maturity term                                               5                                            
Line of credit facility, expiration date                 December 11, 2012                               June 14, 2016                                          
Line of credit facility, unused capacity, commitment fee percentage                                                 0.50%                                          
Weighted-average interest rate 3.04%                                                                                          
Secured revolving credit facility maximum borrowing amount                                                 234,200,000 132,000,000                                        
Letters of credit outstanding, amount 146,600,000                                                                                          
Incurred financing fees 90,500,000 37,600,000                                     14,500,000       21,100,000                                          
Line of credit borrowings outstanding               375,000,000 106,300,000                                                                          
Average borrowings                                                 234,200,000 132,000,000                                        
Line of credit facility, remaining borrowing capacity 901,400,000                                                                                          
Due date of senior notes         Jun. 15, 2019                           Jul. 01, 2013   Aug. 01, 2016 Oct. 01, 2020                                                
Amount of debt redeemed                                   500,000,000                                                        
Redemption premium on extinguishment of debt                                       5,600,000                                                    
Write off of Deferred Debt Issuance Cost                                       3,300,000                                                    
Debt Instrument, Unamortized Premium                                       2,100,000                                                    
Debt instrument, frequency of periodic payment             June 15 and December 15 of each year.                       Interest is payable on the notes on January 1 and July 1 of each year.   Interest is payable on the notes on February 1 and August 1 of each year April 1 and October 1 of each year                                                
Face value of senior notes                                     450,000,000   600,000,000                                                  
Debt instrument redemption period                                                           June 15, 2015 through June 14, 2016 June 15, 2016 through June 14, 2017 June 15, 2016 through June 14, 2017 on June 15, 2017 on June 15, 2019 June 15, 2017 through June 14, 2018 June 15, 2018 through June 14, 2019 between August 1, 2013 and July 31, 2014 between August 1, 2014 and July 31, 2015           prior to October 1, 2013 prior to August 1, 2012  
Debt instrument initial issue price percentage of face amount                                     104.75%   97.464%                                                  
Debt Instrument covenant terms                                   The terms of the senior notes contain restrictive covenants that limit Arch Western's ability to, among other things, incur additional debt, sell or transfer assets, and make certain investments.                                                      
Debt instrument, issuer Arch Western Finance LLC ("Arch Western Finance"                                                                                          
Call premium                                         108.75%           107.00% 107.25%   103.50% 101.75% 103.625% 100.00% 100.00% 102.417% 101.208% 104.375% 102.188% 100.00% 103.625% 102.417% 101.208% 100.00% 107.25%    
Principal Outstanding Issued At Premium                                     118,400,000 118,400,000                                                    
Percentage of maximum redemption aggregate principal amount from equity proceeds                                         35.00%               35.00%                             35.00%    
Tender offer price upon change in control             101.00%                                                                              
Issue costs related to notes             44,200,000                                                                              
Expected aggregate maturities in 2012 280,900,000                                                                                          
Expected aggregate maturities in 2013 672,400,000                                                                                          
Expected aggregate maturities in 2014 0                                                                                          
Expected aggregate maturities in 2015 0                                                                                          
Expected aggregate maturities in 2016 $ 600,000,000                                                                                          
Registration statement effective date deadline             June 13, 2012                                                                              
Registration Rights agreement violation additional per annum penalty             0.25%                                                                              
Registration Rights agreement violation maximum per annum penalty             1.00%                                                                              
Registration Rights agreement violation per annum penalty in first 90 days             0.25%                                                                              
Failed to file registration statement, interest rate one-quarter of one percent