Summary of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) State Subsidiary	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2020 USD ($)
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	74.00%
Percentage of private passenger automobile premiums written in California	88.00%	87.00%	85.00%
Percentage by which dividend income on non redeemable preferred stock, partnership, common stock is partially tax-sheltered	50.00%
Insurance companies security deposits	$ 16,000,000	$ 15,000,000
Impairment charges	0		$ 0
Goodwill impairment loss	0
Premiums written, net	3,730,000,000	3,500,000,000	3,220,000,000
Reinsurance policy, coverage per occurrence after retention limit	600,000,000
Reinsurance policy, retention limit	40,000,000
Ceded premiums	56,725,000	48,941,000	26,881,000
Losses and loss adjustment expenses, ceded	$ (71,000,000)	257,000,000	$ 90,000,000
Vesting term	4 years
Revenue from contract with customer	$ 16,300,000	$ 16,000,000.0
Percentage of revenue	0.40%	0.50%
Cost of goods and services sold	$ 10,500,000	$ 10,500,000
Commission income receivable	1,200,000	1,200,000
Refund liability	$ 700,000	$ 700,000,000
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
Subsequent Event [Member] | Retained Earnings [Member] | Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment				$ 2,000,000