Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2020 USD ($) issue policy		Dec. 31, 2019 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		281		276
Number of issues | issue	[1]	119		112
Remaining weighted average contract period (in years)		7 years 4 months 24 days		7 years 8 months 12 days
Principal	[2]	$ 4,847		$ 5,588
Interest	[2]	3,469		3,885
Total		8,316		9,473
Gross claim liability	[3]	1,088		965
Less: Gross potential recoveries	[4]	1,947		2,184
Discount, net	[5]	(173)		(453)
Net claim liability (recoverable)		(686)		(766)
Unearned premium revenue		45		48
Reinsurance recoverables on paid and unpaid losses	[6]	$ 11		$ 19
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		46		45
Number of issues | issue	[1]	16		13
Remaining weighted average contract period (in years)		6 years 4 months 24 days		7 years 3 months 18 days
Principal		$ 1,422	[2]	$ 1,546
Interest	[2]	1,974		2,107
Total		3,396		3,653
Gross claim liability	[3]	0		0
Less: Gross potential recoveries	[4]	0		0
Discount, net	[5]	0		0
Net claim liability (recoverable)		0		0
Unearned premium revenue		$ 10		$ 6
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		16		19
Number of issues | issue	[1]	3		5
Remaining weighted average contract period (in years)		6 years 4 months 24 days		7 years 2 months 12 days
Principal	[2]	$ 123		$ 248
Interest	[2]	54		110
Total		177		358
Gross claim liability	[3]	0		0
Less: Gross potential recoveries	[4]	0		0
Discount, net	[5]	0		0
Net claim liability (recoverable)		0		0
Unearned premium revenue		$ 0		$ 3
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		0		0
Number of issues | issue	[1]	0		0
Principal	[2]	$ 0		$ 0
Interest	[2]	0		0
Total		0		0
Gross claim liability	[3]	0		0
Less: Gross potential recoveries	[4]	0		0
Discount, net	[5]	0		0
Net claim liability (recoverable)		0		0
Unearned premium revenue		$ 0		$ 0
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		219		212
Number of issues | issue	[1]	100		94
Remaining weighted average contract period (in years)		7 years 10 months 24 days		7 years 10 months 24 days
Principal	[2]	$ 3,302		$ 3,794
Interest	[2]	1,441		1,668
Total		4,743		5,462
Gross claim liability	[3]	1,088		965
Less: Gross potential recoveries	[4]	1,947		2,184
Discount, net	[5]	(173)		(453)
Net claim liability (recoverable)		(686)		(766)
Unearned premium revenue		$ 35		$ 39

[1] An “issue” represents the aggregate of financial guarantee policies that share the same revenue source for purposes of making debt service payments on the insured debt.

[2] Represents contractual principal and interest payments due by the issuer of the obligations insured by MBIA.

[3] The gross claim liability with respect to Puerto Rico exposures are net of expected recoveries for policies in a net payable position.

[4] Gross potential recoveries with respect to certain Puerto Rico exposures are net of the claim liability for policies in a net recoverable position.

[5] Represents discount related to Gross Claim Liability and Gross Potential Recoveries.

[6] Included in “Other assets” on the Company’s consolidated balance sheets.