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Employee Benefit Plans - Additional Information (Detail)
3 Months Ended 12 Months Ended
Sep. 30, 2015
USD ($)
Employee
Sep. 30, 2019
Sep. 30, 2018
USD ($)
Program
$ / shares
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
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     $ 6,700,000 $ 6,300,000 $ 6,000,000  
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     $ 116,436,000 $ 108,449,000    
Other long-term liabilities     24,012,000 23,659,000    
Defined benefit plan funded status included in deferred charges and other assets     2,400,000      
Defined benefit plan funded status included in other long-term liabilities       $ 100,000    
Net actuarial loss balance in accumulated other comprehensive income to be amortized     16,300,000      
Amount to be amortized from accumulated other comprehensive income     $ 1,800,000      
Discount rate to determine periodic pension expense     4.15% 3.60% 3.30%  
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     $ 19,000,000      
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   4.70%        
Renewal within Next Fiscal Year            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Multiemployer pension plans, coverage of bargaining agreement, percentage     9.00%      
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     $ 1,199,000 $ 963,000 $ 795,000  
Management Incentive Compensation Plan            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Partnership incentive distribution     600,000 481,000 397,000  
Management Incentive Compensation Plan Executive Officer            
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]            
Partnership incentive distribution     $ 267,082 214,378 177,034  
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 $ 700,000 $ 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     17,100,000      
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     $ 21,400,000      
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%