Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2018 USD ($) issue policy	Dec. 31, 2017 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		363	375
Number of issues | issue	[1]	137	144
Remaining weighted average contract period (in years)		8 years 9 months 18 days	8 years 10 months 24 days
Principal	[2]	$ 8,341	$ 8,964
Interest	[2]	8,142	8,481
Total	[2]	16,483	17,445
Gross claim liability	[3]	1,128	1,082
Less: Gross potential recoveries	[4]	1,793	782
Discount, net	[5]	(355)	(178)
Net claim liability (recoverable)		(310)	478
Unearned premium revenue		83	90
Reinsurance recoverables on paid and unpaid losses	[6]	$ 19	$ 17
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		66	89
Number of issues | issue	[1]	17	20
Remaining weighted average contract period (in years)		7 years 2 months 12 days	7 years 4 months 24 days
Principal	[2]	$ 2,075	$ 2,764
Interest	[2]	2,374	2,676
Total	[2]	4,449	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 2 months 12 days	4 years 3 months 18 days
Principal	[2]	$ 262	$ 13
Interest	[2]	132	3
Total	[2]	394	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		277	280
Number of issues | issue	[1]	114	119
Remaining weighted average contract period (in years)		9 years 4 months 24 days	9 years 8 months 12 days
Principal	[2]	$ 6,004	$ 6,083
Interest	[2]	5,636	5,756
Total	[2]	11,640	11,839
Gross claim liability	[3]	1,128	1,082
Less: Gross potential recoveries	[4]	1,793	782
Discount, net	[5]	(355)	(178)
Net claim liability (recoverable)		(310)	478
Unearned premium revenue		$ 73	$ 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.