Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2023 USD ($) State Subsidiary	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of insurance companies | Subsidiary	12
Number of states in which company operates | State	11
Percentage of direct premiums written as private passenger automobile lines of insurance	62.00%
Percentage of private passenger automobile premiums written in California	82.00%	82.00%	87.00%
Other comprehensive income (loss)	$ 0	$ 0	$ 0
Percentage of portfolio of unadjusted fair values obtained	98.40%
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	14,000,000
Impairment charges	0	0	0
Goodwill impairment loss	0	0
Net premiums written	4,460,000,000	3,980,000,000	3,860,000,000
Reinsurance reimbursable amount	$ 30,000,000
Reinsurance retention policy, threshold loss ratio	73.50%
Reinsurance policy, coverage per occurrence after retention limit	$ 1,111,000,000
Reinsurance policy, retention limit	100,000,000
Ceded to other companies	(109,445,000)	(80,950,000)	(65,017,000)
Losses and loss adjustment expenses, assumed and ceded	$ 10,000,000	(13,000,000)	(10,000,000)
Vesting term	4 years
Revenue from contract with customer	$ 20,800,000	18,200,000	20,200,000
Cost of goods and services sold	11,400,000	11,000,000	12,800,000
Commission income receivable	1,600,000	1,300,000
Refund liability	1,000,000	800,000
Contract with customer, asset	0	0
Revenue, remaining performance obligation	0	0
Capitalized contract cost, net	200,000	1,100,000
Capitalized contract cost, accumulated amortization	(5,500,000)	(4,700,000)
Capitalized contract cost, amortization	$ 900,000	2,800,000	$ 3,900,000
Software
Summary Of Significant Accounting Policies [Line Items]
Useful life, in years	7 years
Maximum | Building
Summary Of Significant Accounting Policies [Line Items]
Useful life, in years	40 years
Maximum | Furniture Equipment And Purchase Software
Summary Of Significant Accounting Policies [Line Items]
Useful life, in years	7 years
Minimum | Furniture Equipment And Purchase Software
Summary Of Significant Accounting Policies [Line Items]
Useful life, in years	3 years
Private Equity Funds
Summary Of Significant Accounting Policies [Line Items]
Unfunded commitments	$ 8,000,000	$ 9,000,000