Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2021 USD ($) issue policy	Dec. 31, 2020 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		264	281
Number of issues | issue	[1]	108	119
Remaining weighted average contract period (in years)		7 years 6 months	7 years 4 months 24 days
Principal	[2]	$ 4,228	$ 4,847
Interest	[2]	3,193	3,469
Total	[2]	7,421	8,316
Gross claim liability	[3]	897	1,088
Less: Gross potential recoveries	[4]	1,831	1,947
Discount, net	[5]	(242)	(173)
Net claim liability (recoverable)		(692)	(686)
Unearned premium revenue		38	45
Reinsurance recoverables on paid and unpaid losses	[6]	$ 8	$ 11
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		58	46
Number of issues | issue	[1]	17	16
Remaining weighted average contract period (in years)		5 years 10 months 24 days	6 years 4 months 24 days
Principal	[2]	$ 1,480	$ 1,422
Interest	[2]	1,952	1,974
Total	[2]	3,432	3,396
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		$ 8	$ 10
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		3	16
Number of issues | issue	[1]	2	3
Remaining weighted average contract period (in years)		2 years 2 months 12 days	6 years 4 months 24 days
Principal	[2]	$ 7	$ 123
Interest	[2]	1	54
Total	[2]	8	177
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
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	[2]	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		203	219
Number of issues | issue	[1]	89	100
Remaining weighted average contract period (in years)		8 years 4 months 24 days	7 years 10 months 24 days
Principal	[2]	$ 2,741	$ 3,302
Interest	[2]	1,240	1,441
Total	[2]	3,981	4,743
Gross claim liability	[3]	897	1,088
Less: Gross potential recoveries	[4]	1,831	1,947
Discount, net	[5]	(242)	(173)
Net claim liability (recoverable)		(692)	(686)
Unearned premium revenue		$ 30	$ 35

[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.