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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2024 USD ($) Subsidiary State | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |
Number of insurance companies | Subsidiary | 12 |  |
Number of states in which company operates | State | 11 |  |
Percentage of direct premiums written as private passenger automobile lines of insurance | 61.00% | 61.00% | 61.00%
Percentage of private passenger automobile premiums written in California | 84.00% | 82.00% | 82.00%
Other comprehensive income (loss) | $ 0 | $ 0 | $ 0
Percentage of portfolio of unadjusted fair values obtained | 98.40% |  |
Percentage by which dividend income on non redeemable preferred stock, partnership, common stock is partially tax-sheltered | 50.00% |  |
Insurance companies security deposits | $ 15,000,000 | 15,000,000 |
Impairment charges | 0 | 0 | 0
Goodwill and other intangible asset impairment | 0 | 0 |
Net premiums written | 5,380,000,000 | 4,460,000,000 | 3,980,000,000
Reinsurance reimbursable amount | $ 30,000,000 |  |
Reinsurance retention policy, threshold loss ratio | 73.50% |  |
Reinsurance policy, coverage per occurrence after retention limit | $ 1,290,000,000 |  |
Reinsurance policy, retention limit | 150,000,000 |  |
Ceded premiums earned | 136,698,000 | 109,445,000 | 80,950,000
Losses and loss adjustment expenses, assumed and ceded | $ (3,000,000) | 10,000,000 | (13,000,000)
Vesting term (in years) | 4 years |  |
Revenue from contract with customer | $ 25,200,000 | 20,800,000 | 18,200,000
Cost of goods and services sold | 13,400,000 | 11,400,000 | 11,000,000.0
Commission income receivable | 1,700,000 | 1,600,000 |
Refund liability | 1,100,000 | 1,000,000.0 |
Contract with customer, asset | 0 | 0 |
Contract with customer, liability | 0 | 0 |
Revenue, remaining performance obligation | 0 | 0 |
Capitalized contract cost, net | 0 | 200,000 |
Capitalized contract cost, accumulated amortization | 5,700,000 | 5,500,000 |
Capitalized contract cost, amortization | $ 200,000 | 900,000 | $ 2,800,000
Software |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life (in years) | 7 years |  |
Maximum | Building |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life (in years) | 40 years |  |
Maximum | Furniture Equipment And Purchase Software |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life (in years) | 7 years |  |
Minimum | Furniture Equipment And Purchase Software |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Useful life (in years) | 3 years |  |
Private Equity Funds |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Unfunded commitments | $ 5,000,000 | $ 8,000,000 |

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