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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Oct. 01, 2019
Sep. 30, 2016
Schedule Of Significant Accounting Policies [Line Items]              
Cash, cash equivalents, and restricted cash     $ 5,149,000 $ 14,781,000 $ 52,708,000   $ 139,188,000
Cash and cash equivalents     4,899,000 14,531,000      
Restricted cash     250,000 250,000      
Captive insurance collateral, debt securities [1]     58,490,000 45,419,000      
Other income, net [2]       7,043,000      
Gain on sale of security customer account, cash portion [3]       6,824,000      
Recognition of unamortized deferred service liabilities       400,000      
Other expenses       200,000      
Advertising expenses     14,300,000 15,100,000 $ 15,100,000    
Additional payment obligated to pay if degree days exceed ten year average     $ 5,000,000        
ASU 2014-09              
Schedule Of Significant Accounting Policies [Line Items]              
Revenue, practical expedient, financing component [true false]     true        
Scenario Forecast              
Schedule Of Significant Accounting Policies [Line Items]              
Derivative maximum payout $ 5,000,000 $ 5,000,000          
Financial Products Corporation | Delivery and branch expenses              
Schedule Of Significant Accounting Policies [Line Items]              
Charge on weather hedge contract     $ 2,100,000 1,900,000      
Captive Insurance Collateral | Level 1              
Schedule Of Significant Accounting Policies [Line Items]              
Captive insurance collateral, debt securities     58,000,000 44,800,000      
Captive Insurance Collateral | Mutual Funds              
Schedule Of Significant Accounting Policies [Line Items]              
Captive insurance collateral, net asset value     $ 500,000 $ 600,000      
Maximum              
Schedule Of Significant Accounting Policies [Line Items]              
Cash equivalents, highly liquid investments maturity     3 months        
Property and equipment, estimated useful lives     30 years        
Maximum | Subsequent Event              
Schedule Of Significant Accounting Policies [Line Items]              
Right-of-use assets           $ 120,000,000  
Lease liabilities           120,000,000  
Maximum | Scenario Forecast              
Schedule Of Significant Accounting Policies [Line Items]              
Derivative maximum receivable $ 12,500,000 $ 12,500,000          
Maximum | Customer Lists              
Schedule Of Significant Accounting Policies [Line Items]              
Intangible assets, amortization period     10 years        
Maximum | Trade Names              
Schedule Of Significant Accounting Policies [Line Items]              
Intangible assets, amortization period     20 years        
Minimum              
Schedule Of Significant Accounting Policies [Line Items]              
Property and equipment, estimated useful lives     3 years        
Minimum | Subsequent Event              
Schedule Of Significant Accounting Policies [Line Items]              
Right-of-use assets           100,000,000  
Lease liabilities           $ 100,000,000  
Minimum | Customer Lists              
Schedule Of Significant Accounting Policies [Line Items]              
Intangible assets, amortization period     7 years        
Minimum | Trade Names              
Schedule Of Significant Accounting Policies [Line Items]              
Intangible assets, amortization period     3 years        
[1] See Note 2 – Summary of Significant Accounting Policies - Captive Insurance Collateral.
[2] See Note 2 – Summary of Significant Accounting Policies - Other Income, Net.
[3] See Note 2 – Summary of Significant Accounting Policies - Other income, net.