Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Sep. 30, 2017
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		$ 61,924	$ 65,676
Corporate and U.S. government bond fund
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	90.00%	80.00%
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]	$ 55,908	$ 52,735
U.S. large-cap equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	7.00%	14.00%
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]	$ 4,566	$ 9,270
International equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	2.00%	5.00%
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]	$ 1,129	$ 3,063
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		$ 321	$ 608

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