Quarterly Distribution of Available Cash - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
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	15.00%
Fixed charge coverage ratio that must be maintained to make distributions	1.15
Annual cash distributions declared per common unit	$ 0.515	$ 0.485	$ 0.455
General partner incentive distributions exclusive of amounts paid subject to management incentive plan	$ 0.8	$ 0.7	$ 0.6