Fair Value Of Financial Instruments (Aggregate Fair Value And Remaining Contractual Principal Balance Outstanding On Fair Value Option) (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable and other instruments, contractual outstanding principal	$ 318	$ 321
Loans receivable and other instruments, fair value	206	172
Loans receivable and other instruments, difference	112	149
Residential Mortgage Loans [Member] \| Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable and other instruments, contractual outstanding principal	165	168
Loans receivable and other instruments, 90 days or more past due, contractual outstanding principal	153	153
Loans receivable and other instruments, fair value	165	164
Loans receivable and other instruments, 90 days or more past due, fair value	41	8
Loans receivable and other instruments, difference	0	4
Loans receivable and other instruments, 90 days or more past due, difference	112	145
Corporate Loans and Other Instruments [Member] \| Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Loans receivable and other instruments, 90 days or more past due, contractual outstanding principal	0	0
Loans receivable and other instruments, 90 days or more past due, fair value	0	0
Loans receivable and other instruments, 90 days or more past due, difference	0	0
Medium Term Notes [Member] \| Non Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Long-term debt instruments, contractual outstanding principal	115	114
Long-term debt instruments, fair value	106	102
Long-term debt instruments, difference	9	12
Variable Interest Entity Notes [Member] \| Variable Interest Entity [Member]
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items]
Long-term debt instruments, contractual outstanding principal	1,292	1,295
Long-term debt instruments, fair value	1,325	480
Long-term debt instruments, difference	$ (33)	$ 815

Fair Value Option Aggregate Fair Value Loans Receivable

No definition available.

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Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
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- Definition

Fair Value Option Aggregate Fair Value Long Term Debt Instruments

+ References

No definition available.

+ Details

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Balance Type:	credit
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- Definition

Fair Value Option Contractual Outstanding Principal Loans Receivable

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Fair Value Option Contractual Outstanding Principal Long-Term Debt Instruments

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

Fair Value Option Contractual Outstanding Principal Loans Receivable 90 Days or More Past Due

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
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Period Type:	instant

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of loans and long-term receivables (other than securities categorized as trading, available-for-sale or held-to-maturity) that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

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Balance Type:	debit
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- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of long-term debt instruments that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

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- Definition

This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

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- Definition

For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (e)(3)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14064-108612

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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