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, 2018 USD ($) issue policy	Dec. 31, 2017 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		319	375
Number of issues | issue	[1]	131	144
Remaining weighted average contract period (in years)		9 years	8 years 10 months 24 days
Principal	[2]	$ 7,660	$ 8,964
Interest	[2]	7,888	8,481
Total	[2]	15,548	17,445
Gross claim liability	[3]	1,076	1,082
Less: Gross potential recoveries	[4]	2,058	782
Discount, net	[5]	(483)	(178)
Net claim liability (recoverable)		(499)	478
Unearned premium revenue		75	90
Reinsurance recoverables on paid and unpaid losses	[6]	$ 23	$ 17
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		56	89
Number of issues | issue	[1]	16	20
Remaining weighted average contract period (in years)		7 years 2 months 12 days	7 years 4 months 24 days
Principal	[2]	$ 1,933	$ 2,764
Interest	[2]	2,314	2,676
Total	[2]	4,247	5,440
Gross claim liability	[3]	0	0
Less: Gross potential recoveries		0	0
Discount, net	[5]	0	0
Net claim liability (recoverable)		0	0
Unearned premium revenue		$ 6	$ 9
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		20	5
Number of issues | issue	[1]	6	4
Remaining weighted average contract period (in years)		8 years	4 years 3 months 18 days
Principal	[2]	$ 262	$ 13
Interest	[2]	131	3
Total	[2]	393	16
Gross claim liability	[3]	0	0
Less: Gross potential recoveries		0	0
Discount, net	[5]	0	0
Net claim liability (recoverable)		0	0
Unearned premium revenue		$ 4	$ 0
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		0	1
Number of issues | issue	[1]	0	1
Remaining weighted average contract period (in years)			8 years 8 months 12 days
Principal	[2]	$ 0	$ 104
Interest	[2]	0	46
Total	[2]	0	150
Gross claim liability	[3]	0	0
Less: Gross potential recoveries		0	0
Discount, net	[5]	0	0
Net claim liability (recoverable)		0	0
Unearned premium revenue		$ 0	$ 4
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		243	280
Number of issues | issue	[1]	109	119
Remaining weighted average contract period (in years)		9 years 8 months 12 days	9 years 8 months 12 days
Principal	[2]	$ 5,465	$ 6,083
Interest	[2]	5,443	5,756
Total	[2]	10,908	11,839
Gross claim liability	[3]	1,076	1,082
Less: Gross potential recoveries	[4]	2,058	782
Discount, net	[5]	(483)	(178)
Net claim liability (recoverable)		(499)	478
Unearned premium revenue		$ 65	$ 77

[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 sheet.