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Fair Values and Percentage of Company's Pension Plan Assets by Asset Category (Detail) - USD ($) $ in Thousands | Sep. 30, 2017 | Sep. 30, 2016
--- | --- | ---
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 |  | $ 65,676 | $ 68,276
Corporate and U.S. government bond fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of the Partnership's pension plan assets | [1] | 80.00% | 79.00%
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] | $ 52,735 | $ 54,267
U.S. large-cap equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of the Partnership's pension plan assets | [1] | 14.00% | 15.00%
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] | $ 9,270 | $ 10,207
International equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Percentage of the Partnership's pension plan assets | [1] | 5.00% | 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] | $ 3,063 | $ 3,118
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 |  | $ 608 | $ 684
[1] | Represent investments in Vanguard funds that seek to replicate the asset category description.

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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