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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 9	$ 17
Trading Securities, Realized Gain (Loss)	1	5
Investment expense	(1)	(1)	$ (3)	$ (3)
Securities available-for-sale and short-term	22	26	44	57
Net investment income	42	52	91	31
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			16	(16)
Trading Securities, Realized Gain (Loss)			1	(19)
Other Investments Income	8	12	15	4
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	23	26	46	55
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	0	1	0	4
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 20	$ 27	$ 47	$ (25)