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, 2019 USD ($) issue policy	Dec. 31, 2018 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		294	301
Number of issues | issue	[1]	118	122
Remaining weighted average contract period (in years)		7 years 4 months 24 days	8 years 10 months 24 days
Principal	[2]	$ 6,043	$ 7,206
Interest	[2]	3,971	7,655
Total	[2]	10,014	14,861
Gross claim liability	[3]	928	977
Less: Gross potential recoveries	[4]	1,999	2,255
Discount, net	[5]	(300)	(670)
Net claim liability (recoverable)		(771)	(608)
Unearned premium revenue		50	72
Reinsurance recoverables on paid and unpaid losses	[6]	$ 19	$ 21
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		47	50
Number of issues | issue	[1]	15	16
Remaining weighted average contract period (in years)		6 years 7 months 6 days	6 years 8 months 12 days
Principal	[2]	$ 1,527	$ 1,604
Interest	[2]	2,068	2,118
Total	[2]	3,595	3,722
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		$ 5	$ 5
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		18	18
Number of issues | issue	[1]	4	4
Remaining weighted average contract period (in years)		7 years 9 months 18 days	8 years
Principal	[2]	$ 249	$ 249
Interest	[2]	122	123
Total	[2]	371	372
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	$ 4
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
Remaining weighted average contract period (in years)		0 years	0 years
Principal	[2]	$ 0	$ 0
Interest	[2]	0	0
Total	[2]	0	0
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	$ 0
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		229	233
Number of issues | issue	[1]	99	102
Remaining weighted average contract period (in years)		7 years 7 months 6 days	9 years 8 months 12 days
Principal	[2]	$ 4,267	$ 5,353
Interest	[2]	1,781	5,414
Total	[2]	6,048	10,767
Gross claim liability	[3]	928	977
Less: Gross potential recoveries	[4]	1,999	2,255
Discount, net	[5]	(300)	(670)
Net claim liability (recoverable)		(771)	(608)
Unearned premium revenue		$ 41	$ 63

[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.