Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ 17	$ 7
Trading Securities, Realized Gain (Loss)	5	3
Investment expense	(1)	(1)	$ (3)	$ (3)
Securities available-for-sale and short-term	26	78	57	125
Net investment income	52	86	31	141
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities			(16)	14
Trading Securities, Realized Gain (Loss)			(19)	4
Other Investments Income	12	4	4	10
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	26	74	55	118
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	1	5	4	10
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ 27	$ 8	$ (25)	$ 16