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, 2022	Mar. 31, 2021
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ (13)	$ 7
Investment expense	(1)	(1)
Securities available-for-sale and short-term	14	22
Net investment income	5	49
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	(10)	6
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	15	23
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	0	0
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 0	$ 27