Insurance In Force (Summary Of Company's Reinsurance Agreements For U.S. Public Finance And Structured Finance And International Insurance Operations) (Detail) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Aggregate Amount Of Insured Par Outstanding Ceded To Reinsurers		$ 2,724	$ 4,200
Reinsurance Recoverables Secured By Letters Of Credit Or Trust Amount		30
Reinsurance Recoverable	[1]	6
Assured Guaranty Corp [Member]
Aggregate Amount Of Insured Par Outstanding Ceded To Reinsurers		1,031
Reinsurance Recoverable	[1]	3
Assured Guaranty Re Ltd [Member]
Aggregate Amount Of Insured Par Outstanding Ceded To Reinsurers		1,321
Reinsurance Recoverables Secured By Letters Of Credit Or Trust Amount		27
Reinsurance Recoverable	[1]	3
Overseas Private Investment Corporation [Member]
Aggregate Amount Of Insured Par Outstanding Ceded To Reinsurers		281
Others [Member]
Aggregate Amount Of Insured Par Outstanding Ceded To Reinsurers		91
Reinsurance Recoverables Secured By Letters Of Credit Or Trust Amount		$ 3

[1] Total reinsurance recoverable is primarily related to recoverables on unpaid losses.