Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		100.00%	100.00%
Quoted Prices in Active Markets for Identical Assets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets		$ 65,337	$ 69,088
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 | Quoted Prices in Active Markets for Identical Assets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 59,068	$ 62,602
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 | Quoted Prices in Active Markets for Identical Assets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 4,765	$ 5,006
International equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	2.00%	2.00%
International equity | Quoted Prices in Active Markets for Identical Assets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 1,165	$ 1,158
Cash
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		1.00%	1.00%
Cash | Quoted Prices in Active Markets for Identical Assets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets		$ 339	$ 322

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