CONDENSED CONSOLIDATED STATEMENT OF PARTNERS' CAPITAL - USD ($) shares in Thousands, $ in Thousands	Total	General Partner	Common Stock	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Sep. 30, 2019	$ 260,840	$ (1,968)	$ 279,709	$ (16,901)
Beginning Balance, unit at Sep. 30, 2019		326	47,685
Net income	86,163	$ 601	$ 85,562
Unrealized gain on pension plan obligation	910			910
Tax effect of unrealized gain on pension plan obligation	(258)			(258)
Unrealized gain (loss) on captive insurance collateral	43			43
Tax effect of unrealized gain (loss) on captive insurance collateral	(7)			(7)
Unrealized gain (loss) on interest rate hedges	(1,382)			(1,382)
Tax effect of unrealized gain (loss) on interest rate hedges	374			374
Distributions	(12,153)	(425)	(11,728)
Retirement of units	(18,575)		$ (18,575)
Retirement of units, shares			(2,063)
Ending Balance at Mar. 31, 2020	315,955	$ (1,792)	$ 334,968	(17,221)
Ending Balance, Unit at Mar. 31, 2020		326	45,622
Beginning Balance at Dec. 31, 2019	270,898	$ (1,991)	$ 289,268	(16,379)
Beginning Balance, unit at Dec. 31, 2019		326	46,404
Net income	58,408	$ 409	$ 57,999
Unrealized gain on pension plan obligation	455			455
Tax effect of unrealized gain on pension plan obligation	(133)			(133)
Unrealized gain (loss) on captive insurance collateral	107			107
Tax effect of unrealized gain (loss) on captive insurance collateral	(15)			(15)
Unrealized gain (loss) on interest rate hedges	(1,716)			(1,716)
Tax effect of unrealized gain (loss) on interest rate hedges	460			460
Distributions	(5,998)	(210)	(5,788)
Retirement of units	(6,511)		$ (6,511)
Retirement of units, shares			(782)
Ending Balance at Mar. 31, 2020	315,955	$ (1,792)	$ 334,968	(17,221)
Ending Balance, Unit at Mar. 31, 2020		326	45,622
Beginning Balance at Sep. 30, 2020	255,820	$ (2,506)	$ 273,283	(14,957)
Beginning Balance, unit at Sep. 30, 2020		326	43,328
Ending Balance at Dec. 31, 2020	262,616	$ (2,447)	$ 279,873	(14,810)
Ending Balance, Unit at Dec. 31, 2020		326	40,737
Beginning Balance at Sep. 30, 2020	255,820	$ (2,506)	$ 273,283	(14,957)
Beginning Balance, unit at Sep. 30, 2020		326	43,328
Net income	123,024	$ 977	$ 122,047
Unrealized gain on pension plan obligation	470			470
Tax effect of unrealized gain on pension plan obligation	(128)			(128)
Unrealized gain (loss) on captive insurance collateral	(866)			(866)
Tax effect of unrealized gain (loss) on captive insurance collateral	182			182
Unrealized gain (loss) on interest rate hedges	1,051			1,051
Tax effect of unrealized gain (loss) on interest rate hedges	(278)			(278)
Distributions	(11,533)	(462)	(11,071)
Retirement of units	(30,466)		$ (30,466)
Retirement of units, shares			(3,129)
Ending Balance at Mar. 31, 2021	337,276	$ (1,991)	$ 353,793	(14,526)
Ending Balance, Unit at Mar. 31, 2021		326	40,199
Beginning Balance at Dec. 31, 2020	262,616	$ (2,447)	$ 279,873	(14,810)
Beginning Balance, unit at Dec. 31, 2020		326	40,737
Net income	85,164	$ 681	$ 84,483
Unrealized gain on pension plan obligation	235			235
Tax effect of unrealized gain on pension plan obligation	(64)			(64)
Unrealized gain (loss) on captive insurance collateral	(526)			(526)
Tax effect of unrealized gain (loss) on captive insurance collateral	110			110
Unrealized gain (loss) on interest rate hedges	719			719
Tax effect of unrealized gain (loss) on interest rate hedges	(190)			(190)
Distributions	(5,594)	(225)	(5,369)
Retirement of units	(5,194)		$ (5,194)
Retirement of units, shares			(538)
Ending Balance at Mar. 31, 2021	$ 337,276	$ (1,991)	$ 353,793	$ (14,526)
Ending Balance, Unit at Mar. 31, 2021		326	40,199