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Summary of Significant Accounting Policies (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2017 USD ($) State Subsidiary | Dec. 31, 2016 USD ($) | Dec. 31, 2015 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 | 76.00% |  |
Percentage of private passenger automobile premiums written in California | 85.00% | 84.00% | 83.00%
Percentage by which dividend income on non redeemable preferred stock, partnership, common stock is partially tax-sheltered | 70.00% |  |
Insurance companies security deposits | $ 17,000,000 | $ 19,000,000 |
Impairment charges | 0 | 0 | $ 0
Goodwill impairment loss | 0 | 0 |
Premiums written, net | 3,220,000,000 | 3,160,000,000 | 3,000,000,000
Net cash provided by operating activities | 341,405,000 | 291,678,000 | 190,271,000
Net cash provided by financing activities | (84,713,000) | (108,861,000) | (134,275,000)
Operating lease liability | $ 32,000,000 |  |
Options Granted Subsequent To January 1, 2008 [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Option vesting term, years | 4 years |  |
Minimum [Member] | Options Granted Prior To 2008 [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Option vesting term, years | 4 years |  |
Maximum [Member] | Options Granted Prior To 2008 [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Option vesting term, years | 5 years |  |
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 |  |
Accounting Standards Updated 2016-09, Excess Tax Benefit Component [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Net cash provided by operating activities |  | 913,000 |
Net cash provided by financing activities |  |  | 27,000
Accounting Standards Updated 2016-09, Employee Taxes Paid With Shares [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Net cash provided by operating activities |  | $ 3,292,000 |
Net cash provided by financing activities |  |  | $ 0

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