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Summary of Significant Accounting Policies (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2016 USD ($) State Subsidiary | Dec. 31, 2015 USD ($) | Dec. 31, 2014 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 | 77.00% |  |
Percentage of private passenger automobile premiums written in California | 84.00% | 83.00% | 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 | $ 19,000,000 | $ 21,000,000 |
Deferred policy acquisition cost amortization | 562,545,000 | 539,231,000 | $ 526,208,000
Advertising expenses | 40,000,000 | 44,000,000 | 23,000,000
Impairment charges | 0 | 0 | 0
Goodwill impairment loss | 0 | 0 |
Premiums written, net | $ 3,160,000,000 | $ 3,000,000,000 | $ 2,840,000,000
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 |  |

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