Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2025	Sep. 30, 2024
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		100.00%	100.00%
Significant Other Observable Inputs Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets		$ 38,632	$ 41,330
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		$ 4,913	$ 5,660
Corporate and U.S. government bond fund
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	88.70%	88.00%
Corporate and U.S. government bond fund | Significant Other Observable Inputs Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair values of the Partnership's pension plan assets	[1]	$ 38,632	$ 41,330
U.S. large-cap equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	11.00%	12.00%
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]	$ 4,797	$ 5,660
Cash
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets		0.30%	0.00%
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		$ 116	$ 0

[1] Represent investments in Principal funds at September 30, 2025 and September 30, 2024.