Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended	12 Months Ended
		Dec. 31, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Significant Accounting Policies [Line Items]
Cash, cash equivalents, and restricted cash						$ 14,781,000	$ 52,708,000	$ 139,188,000	$ 100,508,000
Cash and cash equivalents						14,531,000	52,458,000
Restricted cash						250,000	250,000
Cash deposit in support of liability		$ 34,200,000
Investments, debt securities	[1]					45,419,000	11,777,000
Other income, net	[2]					7,043,000
Gain on sale of security customer account, cash portion	[2]					6,824,000
Recognition of unamortized deferred service liabilities						400,000
Other expenses						200,000
Advertising expenses						$ 15,100,000	15,100,000	$ 14,900,000
Operating lease expiration year						2033
Undiscounted future minimum lease payments through 2033						$ 127,600,000
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income (Loss)
Schedule Of Significant Accounting Policies [Line Items]
Increase (decrease) in equity as a result of ASU adoption						200,000
Accounting Standards Update 2018-02 | Retained Earnings
Schedule Of Significant Accounting Policies [Line Items]
Increase (decrease) in equity as a result of ASU adoption						(200,000)
Subsidiaries of Swiss Re
Schedule Of Significant Accounting Policies [Line Items]
Additional payment obligated to pay if degree days exceed ten year average						5,000,000
Subsidiaries of Swiss Re | Scenario forecast
Schedule Of Significant Accounting Policies [Line Items]
Derivative maximum payout			$ 5,000,000	$ 5,000,000	$ 5,000,000
Subsidiaries of Swiss Re | Delivery and branch expenses
Schedule Of Significant Accounting Policies [Line Items]
Charge on weather hedge contract						1,900,000
Charge or benefit on weather hedge contract							0
Fair Value Level 1
Schedule Of Significant Accounting Policies [Line Items]
Investments, debt securities						44,800,000	11,300,000
Mutual Funds
Schedule Of Significant Accounting Policies [Line Items]
Investments, net asset value						$ 600,000	$ 500,000
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 | Subsidiaries of Swiss Re | Scenario forecast
Schedule Of Significant Accounting Policies [Line Items]
Derivative maximum receivable			$ 12,500,000	$ 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 | 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 - Investments.

[2] See Note 2 - Other income, net.