Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		100.00%	100.00%
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets		$ 69,088	$ 66,838
Corporate and U.S. government bond fund
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	90.00%	90.00%
Corporate and U.S. government bond fund | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 62,602	$ 60,720
U.S. large-cap equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	7.00%	7.00%
U.S. large-cap equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 5,006	$ 4,632
International equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	2.00%	2.00%
International equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 1,158	$ 1,119
Cash
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		1.00%	1.00%
Cash | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets		$ 322	$ 367

[1] Represent investments in Vanguard funds that seek to replicate the asset category description.