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Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Sep. 30, 2019
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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.

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