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
	Mar. 31, 2020	Mar. 31, 2019
Investment [Line Items]
Net Realized and Unrealized Gain (Loss) on Trading Securities	$ (32)	$ 7
Trading Securities, Realized Gain (Loss)	(3)	1
Investment expense	(2)	(1)
Securities available-for-sale and short-term	31	47
Net investment income (loss)	(21)	55
Gains (losses) on securities held as of reporting date [Member]
Investment [Line Items]
Other Investments Income	(29)	6
Fixed Income Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	30	44
Short-Term [Member]
Investment [Line Items]
Gross Investment Income, Operating	2	4
Loans Receivable [Member]
Investment [Line Items]
Gross Investment Income, Operating	0	0
Other Investments [Member]
Investment [Line Items]
Gross Investment Income, Operating	$ (52)	$ 8