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Summary of Significant Accounting Policies (Narrative) (Details) | 12 Months Ended |  |
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Dec. 31, 2022 USD ($) State Subsidiary | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jan. 01, 2020 USD ($) | Dec. 31, 2019 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 | 64.00% |  |  |  |
Percentage of private passenger automobile premiums written in California | 82.00% | 87.00% | 88.00% |  |
Percentage of portfolio of unadjusted fair values obtained | 98.20% |  |  |  |
Percentage by which dividend income on non redeemable preferred stock, partnership, common stock is partially tax-sheltered | 50.00% |  |  |  |
Insurance companies security deposits | $ 14,000,000 | $ 15,000,000 |  |  |
Impairment charges | 0 | 0 | $ 0 |  |
Goodwill impairment loss | 0 | 0 |  |  |
Net premiums written | 3,980,000,000 | 3,860,000,000 | 3,610,000,000 |  |
Reduction in premiums written, net |  |  | 128,000,000 |  |
Reinsurance reimbursable amount | $ 30,000,000 |  |  |  |
Reinsurance retention policy, threshold loss ratio | 73.50% |  |  |  |
Reinsurance policy, coverage per occurrence after retention limit | $ 936,000,000 |  |  |  |
Reinsurance policy, retention limit | 60,000,000 |  |  |  |
Ceded to other companies | (80,950,000) | (65,017,000) | (56,212,000) |  |
Losses and loss adjustment expenses, ceded | $ (13,000,000) | (10,000,000) | (28,000,000) |  |
Vesting term | 4 years |  |  |  |
Revenue from contract with customer | $ 18,200,000 | 20,200,000 | 18,000,000 |  |
Cost of goods and services sold | 11,000,000 | 12,800,000 | 11,800,000 |  |
Commission income receivable | 1,300,000 | 1,400,000 |  |  |
Refund liability | 800,000 | 900,000 |  |  |
Capitalized contract cost, net | 1,100,000 | 2,800,000 |  |  |
Capitalized contract cost, accumulated amortization | (4,700,000) | (6,000,000) |  |  |
Capitalized contract cost, amortization | 2,800,000 | 3,900,000 | 1,800,000 |  |
Ending retained earnings | 1,522,131,000 | 2,140,281,000 | 2,032,597,000 |  |
Retained Earnings |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Ending retained earnings | $ 1,423,184,000 | 2,041,338,000 | 1,933,627,000 |  | $ 1,700,674,000
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Ending retained earnings |  | 0 | $ 0 |  | $ (2,014,000)
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 | Retained Earnings |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Ending retained earnings |  |  |  | $ 2,000,000 |
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 | $ 9,000,000 | $ 32,000,000 |  |  |

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