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COMPENSATION PLANS - Other Plans (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
AHGP LTIP      
Compensation Plans      
Units reserved for issuance 5,215,000    
SERP and Deferred Compensation Plans      
Compensation Plans      
Common unit-based based compensation expense $ 1.5 $ 1.5 $ 1.5
SERP and Deferred Compensation Plans | Phantom Share Units (PSUs)      
Compensation Plans      
Units outstanding 450,828    
Total intrinsic value $ 6.2    
SERP And MGP Deferred Compensation Plan      
Compensation Plans      
Total unit based obligation recorded $ 13.8 $ 12.6  
SERP And MGP Deferred Compensation Plan | Phantom Share Units (PSUs)      
Compensation Plans      
Units granted 60,160 27,577  
Fair value (in dollars per unit) $ 21.38 $ 44.56  
AGP Deferred Compensation Plan      
Compensation Plans      
Total unit based obligation recorded $ 1.1 $ 1.0  
AGP Deferred Compensation Plan | Phantom Share Units (PSUs)      
Compensation Plans      
Units granted 5,478 4,868  
Fair value (in dollars per unit) $ 40.87 $ 63.73