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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.

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