Fair Values and Percentage of Partnership's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2016	Sep. 30, 2015
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		$ 68,276	$ 63,788
Corporate and U.S. government bond fund
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	79.00%	81.00%
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]	$ 54,267	$ 51,477
U.S. large-cap equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	15.00%	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]	$ 10,207	$ 8,932
International equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of the Partnership's pension plan assets	[1]	5.00%	4.00%
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]	$ 3,118	$ 2,832
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		$ 684	$ 547

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