Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Investment [Line Items]
Investment expense	$ (1)	$ (1)	$ (4)	$ (4)
Net investment income	38	30	116	100
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	421		421		$ 482
Standard & Poor's, B- Rating [Member] | Mortgage-Backed Securities, Other
Investment [Line Items]
Investment Owned, at Fair Value	258		258		240
Standard & Poor's, B- Rating [Member] | Asset-backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	154		154		$ 232
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	9	0	15	6
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	22	20	67	55
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	4	5	14	16
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 14	$ 6	$ 35	$ 28