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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 0	$ (7)
Investment expense	(1)	(2)	$ (4)	$ (4)
Interest and Dividend Income, Securities, Operating, Available-for-Sale	24	17	67	44
Net investment income (loss)	30	11	100	(6)
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			16	(49)
Other Investments Income	0	(6)	6	(22)
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	20	16	55	45
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	5	3	16	4
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 6	$ (5)	$ 28	$ (28)