Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2018 USD ($) issue policy	Dec. 31, 2017 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		367	375
Number of issues | issue	[1]	139	144
Remaining weighted average contract period (in years)		8 years 9 months 18 days	8 years 10 months 24 days
Principal	[2]	$ 8,617	$ 8,964
Interest	[2]	8,248	8,481
Total	[2]	16,865	17,445
Gross claim liability	[3]	1,107	1,082
Less: Gross potential recoveries	[4]	878	782
Discount, net	[5]	(248)	(178)
Net claim liability (recoverable)		477	478
Unearned premium revenue		$ 86	$ 90
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		72	89
Number of issues | issue	[1]	18	20
Remaining weighted average contract period (in years)		7 years	7 years 4 months 24 days
Principal	[2]	$ 2,371	$ 2,764
Interest	[2]	2,451	2,676
Total	[2]	4,822	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		$ 7	$ 9
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		19	5
Number of issues | issue	[1]	5	4
Remaining weighted average contract period (in years)		8 years 6 months	4 years 3 months 18 days
Principal	[2]	$ 159	$ 13
Interest	[2]	93	3
Total	[2]	252	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		$ 1	$ 0
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		1	1
Number of issues | issue	[1]	1	1
Remaining weighted average contract period (in years)		8 years 4 months 24 days	8 years 8 months 12 days
Principal	[2]	$ 104	$ 104
Interest	[2]	46	46
Total	[2]	150	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		$ 3	$ 4
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		275	280
Number of issues | issue	[1]	115	119
Remaining weighted average contract period (in years)		9 years 7 months 6 days	9 years 8 months 12 days
Principal	[2]	$ 5,983	$ 6,083
Interest	[2]	5,658	5,756
Total	[2]	11,641	11,839
Gross claim liability	[3]	1,107	1,082
Less: Gross potential recoveries	[4]	878	782
Discount, net	[5]	(248)	(178)
Net claim liability (recoverable)		477	478
Unearned premium revenue		$ 75	$ 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.