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COMPENSATION PLANS (Details) (USD $)
In Millions, except Share data, unless otherwise specified
0 Months Ended 3 Months Ended 9 Months Ended 0 Months Ended 36 Months Ended 3 Months Ended 9 Months Ended 9 Months Ended 9 Months Ended
Feb. 15, 2013
ARLP LTIP
Sep. 30, 2013
ARLP LTIP
Sep. 30, 2012
ARLP LTIP
Sep. 30, 2013
ARLP LTIP
Sep. 30, 2012
ARLP LTIP
Sep. 30, 2013
ARLP LTIP
Phantom Share Units (PSUs)
Jan. 23, 2013
ARLP LTIP
Phantom Share Units (PSUs)
Jan. 02, 2013
ARLP LTIP, 2010 Grants
Phantom Share Units (PSUs)
Dec. 31, 2012
ARLP LTIP, 2010 Grants
Phantom Share Units (PSUs)
Sep. 30, 2013
AHGP LTIP
Sep. 30, 2013
SERP and Deferred Compensation Plans
Sep. 30, 2012
SERP and Deferred Compensation Plans
Sep. 30, 2013
SERP and Deferred Compensation Plans
Sep. 30, 2012
SERP and Deferred Compensation Plans
Sep. 30, 2013
SERP and Deferred Compensation Plans
Phantom Share Units (PSUs)
Sep. 30, 2013
SERP And MGP Deferred Compensation Plan
Sep. 30, 2013
SERP And MGP Deferred Compensation Plan
Phantom Share Units (PSUs)
Sep. 30, 2012
SERP And MGP Deferred Compensation Plan
Phantom Share Units (PSUs)
Sep. 30, 2013
AGP Deferred Compensation Plan
Sep. 30, 2013
AGP Deferred Compensation Plan
Phantom Share Units (PSUs)
Sep. 30, 2012
AGP Deferred Compensation Plan
Phantom Share Units (PSUs)
Compensation Plans                                          
Units for which vesting requirements were deemed satisfied               130,102                          
Forfeited (in units)                 8,028                        
Common units issued upon vesting 82,400                                        
Common units granted           146,725                     10,835 7,168   4,086 2,950
Additional grants authorized (in units)             156,575                            
Share based compensation expense   $ 1.8 $ 1.6 $ 5.4 $ 4.7           $ 0.4 $ 0.3 $ 1.1 $ 0.8              
Fair value as intrinsic value at date of grant (in dollars per unit)           $ 63.02                     $ 68.32 $ 64.77   $ 54.61 $ 46.28
Units available for grant   2,100,000   2,100,000                                  
Unrecognized compensation expense   10.7   10.7                                  
Weighted-average period for recognition of expense       1 year 4 months 24 days                                  
Intrinsic value of outstanding grants           25.8                 13.4            
Total unit based obligation recorded   $ 12.7   $ 12.7                       $ 11.1     $ 0.8    
Units reserved for issuance under the LTIP                   5,215,000                      
Units outstanding                             184,457