Summary of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	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