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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 3	$ 6
Trading Securities, Realized Gain (Loss)	0	2
Investment expense	(1)	(1)	$ (4)	$ (4)
Securities available-for-sale and short-term	24	36	80	161
Net investment income	37	45	69	186
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			(12)	19
Trading Securities, Realized Gain (Loss)			(19)	4
Other Investments Income	3	4	7	15
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	24	33	79	151
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	1	4	4	14
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 14	$ 8	$ (11)	$ 25