Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written									$ 1	$ 28	$ 24
Assumed Reinsurance Premiums Written									0	0	0
Ceded Reinsurance Premiums Written									1	(31)	(17)
Premiums written, net of reinsurance									0	60	41
Direct Premiums Earned									65	75	119
Premiums Earned, Net									54	66	111
Assumed Reinsurance Premiums Earned									1	0	0
Ceded Reinsurance Premiums Earned									12	10	8
Reinsurance on premiums earned, Net	$ 18	$ 15	$ 11	$ 10	$ 20	$ 10	$ 8	$ 28	$ 54	$ 66	$ 111