Weighted-Average Assumptions Used in Measurement of Partnership's Benefit Obligation (Detail)	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Weighted-Average Assumptions Used in the Measurement of the Company’s Benefit Obligation
Discount rate at year end date	2.45%	3.00%	4.15%
Expected return on plan assets for the year ended	4.36%	4.67%	4.86%

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration