Investments - Summary of Amortized Cost and Fair Values of Fixed Maturity Securities (Details) - USD ($)	Mar. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Cost	$ 32,000,000	$ 33,000,000
Cost, Less than 12 Months	10,000,000	11,000,000
Cost, 12 Months or More	22,000,000	22,000,000
Unrealized losses, 12 Months or More	(1,000,000)	(1,000,000)
Fair value	31,000,000	32,000,000
U.S. Treasury securities, obligations of U.S. Government agencies and municipalities
Debt Securities, Available-for-sale [Line Items]
Cost	24,000,000	25,000,000
Cost, Less than 12 Months	6,000,000	7,000,000
Cost, 12 Months or More	18,000,000	18,000,000
Unrealized losses, 12 Months or More	(1,000,000)	(1,000,000)
Fair value	23,000,000	24,000,000
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Cost	5,000,000	5,000,000
Cost, Less than 12 Months	4,000,000	4,000,000
Cost, 12 Months or More	1,000,000	1,000,000
Fair value	5,000,000	5,000,000
Non-subsidiary equity
Debt Securities, Available-for-sale [Line Items]
Cost	3,000,000	3,000,000
Cost, 12 Months or More	3,000,000	3,000,000
Fair value	$ 3,000,000	$ 3,000,000