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Quarterly Distribution of Available Cash - Additional Information (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
 12 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Sep. 30, 2011
Selected Quarterly Financial Data [Line Items]      
Minimum distribution of available cash per unit $ 0.0675    
Common units distribution amount on annualized basis $ 0.27    
First tier, percentage of cash distribution to common units minimum quarterly distribution 100.00%    
Second tier, percentage of available cash distribution to common units for any arrearages 100.00%    
Third tier, percentage of available cash distribution to general partner unit holder until minimum quarterly distribution is met 100.00%    
Fourth tier, percentage of distributions to common unit holders until first target distribution is met 90.00%    
Fourth tier, percentage of distributions to general partner unit holder until first target distribution is met 10.00%    
First target distribution $ 0.1125    
Thereafter, percentage of distributions to common unit holders after first target distribution is met 80.00%    
Thereafter, percentage of distributions to general partner unit holders after first target distribution is met 20.00%    
Percentage of excess availability of revolving commitment that must be maintained 17.50%    
Fixed charge coverage ratio that must be maintained to make distributions 1.15    
Annual cash distributions declared per common unit $ 0.320 $ 0.310 $ 0.305
General partner incentive distributions exclusive of amounts paid subject to management incentive plan $ 0.2 $ 0.1 $ 0.1