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Summary of Significant Accounting Policies - Narrative (Details) | 12 Months Ended
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Dec. 31, 2023 USD ($) State Subsidiary | Dec. 31, 2022 USD ($) | Dec. 31, 2021 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 | 62.00% |  |
Percentage of private passenger automobile premiums written in California | 82.00% | 82.00% | 87.00%
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 | 14,000,000 |
Impairment charges | 0 | 0 | 0
Goodwill impairment loss | 0 | 0 |
Net premiums written | 4,460,000,000 | 3,980,000,000 | 3,860,000,000
Reinsurance reimbursable amount | $ 30,000,000 |  |
Reinsurance retention policy, threshold loss ratio | 73.50% |  |
Reinsurance policy, coverage per occurrence after retention limit | $ 1,111,000,000 |  |
Reinsurance policy, retention limit | 100,000,000 |  |
Ceded to other companies | (109,445,000) | (80,950,000) | (65,017,000)
Losses and loss adjustment expenses, assumed and ceded | $ 10,000,000 | (13,000,000) | (10,000,000)
Vesting term | 4 years |  |
Revenue from contract with customer | $ 20,800,000 | 18,200,000 | 20,200,000
Cost of goods and services sold | 11,400,000 | 11,000,000 | 12,800,000
Commission income receivable | 1,600,000 | 1,300,000 |
Refund liability | 1,000,000 | 800,000 |
Contract with customer, asset | 0 | 0 |
Revenue, remaining performance obligation | 0 | 0 |
Capitalized contract cost, net | 200,000 | 1,100,000 |
Capitalized contract cost, accumulated amortization | (5,500,000) | (4,700,000) |
Capitalized contract cost, amortization | $ 900,000 | 2,800,000 | $ 3,900,000
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 | $ 8,000,000 | $ 9,000,000 |

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