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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 9	$ (29)
Investment expense	(1)	(1)	$ (3)	$ (3)
Interest and Dividend Income, Securities, Operating, Available-for-Sale	22	13	43	27
Net investment income (loss)	35	(21)	69	(16)
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			16	(42)
Other Investments Income	2	(15)	6	(17)
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	19	13	35	28
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	5	1	10	1
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 9	$ (23)	$ 22	$ (23)