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, 2021	Mar. 31, 2020
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 7	$ (32)
Trading Securities, Realized Gain (Loss)	0	(3)
Investment expense	(1)	(2)
Securities available-for-sale and short-term	22	31
Net investment income (loss)	49	(21)
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	6	(29)
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	23	30
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	0	2
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 27	$ (52)