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