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
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Investment [Line Items]
Investment expense	$ (1)	$ (2)
Net investment income	42	34
Standard & Poor's, B- Rating [Member]
Investment [Line Items]
Investment Owned, at Fair Value	406
Standard & Poor's, B- Rating [Member] | Mortgage-Backed Securities, Other
Investment [Line Items]
Investment Owned, at Fair Value	244
Standard & Poor's, B- Rating [Member] | Asset-backed Securities [Member]
Investment [Line Items]
Investment Owned, at Fair Value	154
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	7	5
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	23	17
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	5	6
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 13	$ 13