CONSOLIDATED STATEMENTS OF PARTNERS' CAPITAL - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	General Partner	General Partner Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Sep. 30, 2017	$ 306,068		$ (929)		$ 325,762		$ (18,765)
Beginning Balance, unit at Sep. 30, 2017			326		55,888
Reclassification of stranded tax effects resulting from tax reform					$ (195)		195
Net income	55,505		$ 314		55,191
Unrealized gain on pension plan obligation	2,075						2,075
Tax effect of unrealized gain on pension plan obligation	(625)						(625)
Unrealized gain (loss) on captive insurance collateral	(1,204)						(1,204)
Tax effect of unrealized gain (loss) on captive insurance collateral	254						254
Unrealized gain (loss) on interest rate hedge	39						39
Tax effect of unrealized gain (loss) on interest rate hedge	(10)						(10)
Distributions	(25,603)		(688)		(24,915)
Retirement of units	(26,714)				$ (26,714)
Retirement of units, shares					(2,800)
Ending Balance at Sep. 30, 2018	309,785	$ 9,224	$ (1,303)	$ 60	$ 329,129	$ 9,164	(18,041)
Ending Balance, Unit at Sep. 30, 2018			326		53,088
Accounting Standards Update Extensible List		us-gaap:AccountingStandardsUpdate201409Member
Net income	17,637		$ 95		$ 17,542
Unrealized gain on pension plan obligation	1,170						1,170
Tax effect of unrealized gain on pension plan obligation	(320)						(320)
Unrealized gain (loss) on captive insurance collateral	2,231						2,231
Tax effect of unrealized gain (loss) on captive insurance collateral	(473)						(473)
Unrealized gain (loss) on interest rate hedge	(1,993)						(1,993)
Tax effect of unrealized gain (loss) on interest rate hedge	525						525
Distributions	(25,593)		(820)		(24,773)
Retirement of units	(51,353)				$ (51,353)
Retirement of units, shares					(5,403)
Ending Balance at Sep. 30, 2019	260,840		$ (1,968)		$ 279,709		(16,901)
Ending Balance, Unit at Sep. 30, 2019			326		47,685
Net income	55,918		$ 377		$ 55,541
Unrealized gain on pension plan obligation	2,876						2,876
Tax effect of unrealized gain on pension plan obligation	(782)						(782)
Unrealized gain (loss) on captive insurance collateral	916						916
Tax effect of unrealized gain (loss) on captive insurance collateral	(190)						(190)
Unrealized gain (loss) on interest rate hedge	(1,193)						(1,193)
Tax effect of unrealized gain (loss) on interest rate hedge	317						317
Distributions	(24,451)		(915)		(23,536)
Retirement of units	(38,431)				$ (38,431)
Retirement of units, shares					(4,357)
Ending Balance at Sep. 30, 2020	$ 255,820		$ (2,506)		$ 273,283		$ (14,957)
Ending Balance, Unit at Sep. 30, 2020			326		43,328