Employee Benefit Plans - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Employee	Sep. 30, 2020	Sep. 30, 2019 USD ($) Program $ / shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2015 USD ($) Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum participant contribution			60.00%
Partnership aggregate contributions to defined contribution plans			$ 7,600,000	$ 6,700,000	$ 6,300,000
Multiemployer pension plans, coverage of bargaining agreement, percentage			43.00%
Maximum funded for red zone			65.00%
Maximum funded for yellow zone			80.00%
Minimum funded for green zone			80.00%
Partnership contributions in excess of 5% of the plan's total contributions in percentage			5.00%	5.00%	5.00%
Accrued expenses and other current liabilities			$ 120,839,000	$ 116,436,000
Other long-term liabilities			25,746,000	24,012,000
Defined benefit plan funded status included in deferred charges and other assets			1,800,000	$ 2,400,000
Net actuarial loss balance in accumulated other comprehensive income to be amortized			15,100,000
Amount to be amortized from accumulated other comprehensive income			$ 1,800,000
Discount rate to determine periodic pension expense			3.00%	4.15%	3.60%
Expected optional pension contribution			$ 0
Expected benefit payments for year one			4,400,000
Expected benefit payments for year two			4,400,000
Expected benefit payments for year three			4,400,000
Expected benefit payments for year four			4,400,000
Expected benefit payments for year five			4,400,000
Aggregate expected benefit payment for five years thereafter			$ 18,900,000
Cash and Cash Equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pension asset allocation			1.00%
Scenario Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Rate pension plan assets expect to earn		4.40%
Renewal within Next Fiscal Year
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Multiemployer pension plans, coverage of bargaining agreement, percentage			23.00%
Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incentive distribution percentage of available cash in excess of minimum quarterly distribution			50.00%
Gains interest cash proceeds percentage from the sale of general partner units			50.00%
Employee and Consultant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Participant's vested percentage of plan benefits			100.00%
Termination of employment or consultation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Participant's vested percentage of plan benefits			20.00%
Chief executive officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Participant's vested percentage of plan benefits			33.333%
Management And General Partner
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Partnership incentive distribution			$ 1,464,000	$ 1,199,000	$ 963,000
Management Incentive Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Partnership incentive distribution			732,000	600,000	481,000
Management Incentive Compensation Plan Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Partnership incentive distribution			$ 397,430	267,082	214,378
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Participant Contribution			0.00%
Minimum | Management
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Quarterly distribution | $ / shares			$ 0.0675
Other Postretirement Benefit Plan, Defined Benefit
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Partnership aggregate contributions to defined contribution plans			$ 600,000	$ 600,000	$ 700,000
New England Teamsters & Trucking Industry Pension Fund
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum funded for red zone	65.00%
Multiemployer plan undiscounted withdrawal obligation	$ 48,000,000					$ 48,000,000
Expected benefit payments in years						30 years
Multiemployer plan annual liability annual payment amount	$ 1,600,000					$ 1,600,000
Accrued expenses and other current liabilities			200,000
Other long-term liabilities			16,900,000
New England Teamsters & Trucking Industry Pension Fund | Significant Other Observable Inputs Level 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Multiemployer plan discounted withdrawal liability			$ 21,100,000
New England Teamsters & Trucking Industry Pension Fund | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employees | Employee	200					200
Frozen Defined Benefit Pension Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of frozen defined benefit pension plans | Program			2
Domestic Fixed Income
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pension asset allocation			90.00%
Domestic Equities
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pension asset allocation			7.00%
International equity
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pension asset allocation			2.00%