Credit Enhancements - Components of Credit Enhancements, Net (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Credit Enhancement [Line Items]
Premiums, amortization, and transaction costs	$ (231)	$ (162)
Recoveries	467	4
Credit enhancement expense	$ 236	$ (158)