Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total premiums earned	$ 73	$ 85	$ 162
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	55	63	94
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	16	17	68
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 5	$ 0