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		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Investment [Line Items]
Investment expense	$ (1)	$ (1)	$ (3)	$ (3)
Net investment income	36	35	78	69
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	403		403
Standard & Poor's, B- Rating [Member] | Mortgage-Backed Securities, Other
Investment [Line Items]
Investment Owned, at Fair Value	244		244
Standard & Poor's, B- Rating [Member] | Asset-backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	151		151
Standard & Poor's, B Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value					$ 482
Standard & Poor's, B Rating [Member] | Mortgage-Backed Securities, Other
Investment [Line Items]
Investment Owned, at Fair Value					240
Standard & Poor's, B Rating [Member] | Asset-backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value					$ 232
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	2	2	9	6
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	22	19	45	35
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	5	5	10	10
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 8	$ 9	$ 21	$ 22