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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.

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