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Summary of Significant Accounting Policies (Narrative) (Details) | 12 Months Ended |  |
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Dec. 31, 2020 USD ($) State Subsidiary | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Jan. 01, 2020 USD ($) | Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Number of insurance companies | Subsidiary | 14 |  |  |  |
Number of states in which company operates | State | 11 |  |  |  |
Percentage of direct premiums written as private passenger automobile lines of insurance | 70.00% |  |  |  |
Percentage of private passenger automobile premiums written in California | 88.00% | 88.00% | 87.00% |  |
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 | $ 16,000,000 |  |  |
Impairment charges | 0 | 0 | $ 0 |  |
Goodwill impairment loss | 0 | 0 |  |  |
Premiums written, net | 3,610,000,000 | 3,730,000,000 | 3,500,000,000 |  |
Reduction in premiums written, net | 128,000,000 |  |  |  |
Reinsurance reimbursable amount | $ 31,000,000 |  |  |  |
Reinsurance retention policy, threshold loss ratio | 71.00% |  |  |  |
Reinsurance policy, coverage per occurrence after retention limit | $ 717,000,000 |  |  |  |
Reinsurance policy, retention limit | 40,000,000 |  |  |  |
Ceded premiums | 56,212,000 | 56,725,000 | 48,941,000 |  |
Losses and loss adjustment expenses, ceded | $ (28,000,000) | (71,000,000) | 257,000,000 |  |
Vesting term | 4 years |  |  |  |
Revenue from contract with customer | $ 18,000,000.0 | 16,300,000 | 16,000,000.0 |  |
Cost of goods and services sold | 11,800,000 | 10,500,000 | 10,500,000 |  |
Commission income receivable | 1,200,000 | 1,200,000 |  |  |
Refund liability | 900,000 | 700,000 |  |  |
Capitalized contract cost, net | 4,800,000 | 4,400,000 |  |  |
Capitalized contract cost, accumulated amortization | (2,100,000) | (300,000) |  |  |
Capitalized contract cost, amortization | 1,800,000 | 300,000 |  |  |
Cumulative effect adoption adjustment | $ 2,032,597,000 | 1,799,502,000 | 1,617,684,000 |  |
Building [Member] | Minimum [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life, in years | 30 years |  |  |  |
Building [Member] | Maximum [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life, in years | 40 years |  |  |  |
Furniture Equipment And Purchase Software [Member] | Minimum [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life, in years | 3 years |  |  |  |
Furniture Equipment And Purchase Software [Member] | Maximum [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life, in years | 7 years |  |  |  |
Software [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Useful life, in years | 7 years |  |  |  |
Retained Earnings [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Cumulative effect adoption adjustment | $ 1,933,627,000 | $ 1,700,674,000 | 1,519,658,000 |  | $ 1,663,864,000
Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Cumulative effect adoption adjustment | $ (2,014,000) |  | $ 0 |  | $ 0
Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13 [Member] |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |
Cumulative effect adoption adjustment |  |  |  | $ 2,000,000 |

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