Financial Guarantee Insurance Contracts - Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]
Direct Premiums Written	$ 11	$ 3
Assumed Reinsurance Premiums Written	0	0
Ceded Reinsurance Premiums Written	(1)	(1)
Premiums written, net of reinsurance	(12)	(4)
Direct Premiums Earned	13	29
Assumed Reinsurance Premiums Earned	1	0
Ceded Reinsurance Premiums Earned	3	2
Reinsurance on premiums earned, Net	$ 10	$ 28

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