Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written									$ 28	$ 24	$ 14
Assumed Reinsurance Premiums Written									0	0	0
Ceded Reinsurance Premiums Written									(31)	(17)	2
Premiums written, net of reinsurance									60	41	12
Direct Premiums Earned									75	119	190
Premiums Earned, Net									66	111	175
Assumed Reinsurance Premiums Earned									0	0	0
Ceded Reinsurance Premiums Earned									10	8	15
Reinsurance on premiums earned, Net	$ 20	$ 10	$ 8	$ 28	$ 29	$ 26	$ 26	$ 31	$ 66	$ 111	$ 175