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Fair Value Of Financial Instruments (Changes In Level 3 Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) - Fair Value Inputs Level 3 [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | $ 669 |  | $ 613 |  | $ 649 |  | $ 637 |
Total gains/(losses) included in earnings, assets |  | 5 |  | 46 |  | 32 |  | 93 |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | (5) |  | (6) |  | (12) |  | (25) |
Sales, assets |  | 0 |  | 0 |  | 0 |  | (49) |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | (3) | [2] | 0 |  | (6) | [2]
Ending balance, fair value assets |  | 669 |  | 650 |  | 669 |  | 650 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | 4 |  | 45 |  | 30 |  | 89 |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Beginning balance, fair value liabilities |  | 432 |  | 489 |  | 500 |  | 513 |
Total (gains)/losses included in earnings, liabilities |  | 19 |  | 20 |  | 15 |  | 72 |
Unrealized (gains)/losses included in OCI, liabilities |  | (6) | [3] | 5 |  | (56) | [3] | 4 |
Purchases, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  | 0 |  | 3 |  | 0 |  | 7 |
Settlements, liabilities |  | (4) |  | (19) |  | (18) |  | (38) |
Sales, liabilities |  | 0 |  | 0 |  | 0 |  | (60) |
Transfers into Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | 441 |  | 498 |  | 441 |  | 498 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  | 17 |  | (4) |  | 9 |  | 35 |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] | (5) |  |  |  | (54) |  |  |
Residential Prime Financing Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | 116 |  | 154 |  | 136 |  | 172 |
Total gains/(losses) included in earnings, assets |  | 2 |  | (3) |  | (11) |  | 40 |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | (4) |  | (5) |  | (11) |  | (18) |
Sales, assets |  | 0 |  | 0 |  | 0 |  | (48) |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value assets |  | 114 |  | 146 |  | 114 |  | 146 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | 1 |  | (3) |  | (13) |  | 35 |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | 524 |  | 428 |  | 486 |  | 418 |
Total gains/(losses) included in earnings, assets |  | 6 |  | 50 |  | 44 |  | 60 |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Sales, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value assets |  | 530 |  | 478 |  | 530 |  | 478 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | 6 |  | 50 |  | 44 |  | 60 |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  |  |  |  |  |  |  | 5 |
Total gains/(losses) included in earnings, assets |  |  |  |  |  |  |  | 0 |
Unrealized gains/(losses) included in OCI, assets |  |  |  |  |  |  |  | 0 |
Purchases, assets |  |  |  |  |  |  |  | 0 |
Issuances, assets |  |  |  |  |  |  |  | 0 |
Settlements, assets |  |  |  |  |  |  |  | (2) |
Sales, assets |  |  |  |  |  |  |  | 0 |
Transfers into level 3, assets | [2] |  |  |  |  |  |  | 0 |
Transfers out of level 3, assets | [2] |  |  |  |  |  |  | (3) |
Ending balance, fair value assets |  |  |  | 0 |  |  |  | 0 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  |  |  |  |  |  |  | 0 |
Commercial Mortgage Backed Securities [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  |  |  | 4 |  |  |  | 7 |
Total gains/(losses) included in earnings, assets |  |  |  | 0 |  |  |  | 0 |
Unrealized gains/(losses) included in OCI, assets |  |  |  | 0 |  |  |  | 0 |
Purchases, assets |  |  |  | 0 |  |  |  | 0 |
Issuances, assets |  |  |  | 0 |  |  |  | 0 |
Settlements, assets |  |  |  | (1) |  |  |  | (4) |
Sales, assets |  |  |  | 0 |  |  |  | 0 |
Transfers into level 3, assets | [2] |  |  | 0 |  |  |  | 0 |
Transfers out of level 3, assets | [2] |  |  | (3) |  |  |  | (3) |
Ending balance, fair value assets |  |  |  | 0 |  |  |  | 0 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  |  |  | 0 |  |  |  | 0 |
Collateralized Debt Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  |  |  |  |  |  |  | 1 |
Total gains/(losses) included in earnings, assets |  |  |  |  |  |  |  | 0 |
Unrealized gains/(losses) included in OCI, assets |  |  |  |  |  |  |  | 0 |
Purchases, assets |  |  |  |  |  |  |  | 0 |
Issuances, assets |  |  |  |  |  |  |  | 0 |
Settlements, assets |  |  |  |  |  |  |  | 0 |
Sales, assets |  |  |  |  |  |  |  | (1) |
Transfers into level 3, assets | [2] |  |  |  |  |  |  | 0 |
Transfers out of level 3, assets | [2] |  |  |  |  |  |  | 0 |
Ending balance, fair value assets |  |  |  | 0 |  |  |  | 0 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  |  |  |  |  |  |  | 0 |
Other Asset Backed [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | 1 |  | 1 |  | 1 |  | 3 |
Total gains/(losses) included in earnings, assets |  | 0 |  | 0 |  | 0 |  | (1) |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | (1) |  | 0 |  | (1) |  | (1) |
Sales, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value assets |  | 0 |  | 1 |  | 0 |  | 1 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Medium Term Notes [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value liabilities |  | 96 |  | 110 |  | 108 |  | 102 |
Total (gains)/losses included in earnings, liabilities |  | 8 |  | 1 |  | 8 |  | 6 |
Unrealized (gains)/losses included in OCI, liabilities |  | (3) | [3] | (5) |  | (15) | [3] | 1 |
Purchases, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, liabilities |  | 0 |  | 0 |  | 0 |  | (3) |
Sales, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | 101 |  | 106 |  | 101 |  | 106 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  | 8 |  | 1 |  | 8 |  | 1 |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] | (3) |  |  |  | (15) |  |  |
Credit Derivatives [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value liabilities |  | 8 |  | 18 |  | 7 |  | 33 |
Total (gains)/losses included in earnings, liabilities |  | 0 |  | 1 |  | 1 |  | (12) |
Unrealized (gains)/losses included in OCI, liabilities |  | 0 | [3] | 0 |  | 0 | [3] | 0 |
Purchases, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, liabilities |  | 0 |  | (9) |  | 0 |  | (11) |
Sales, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | 8 |  | 10 |  | 8 |  | 10 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  | 0 |  | (9) |  | 1 |  | (23) |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] | 0 |  |  |  | 0 |  |  |
Other Derivatives Liabilities [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value liabilities |  | 37 |  | 16 |  | 34 |  | 7 |
Total (gains)/losses included in earnings, liabilities |  | 1 |  | 14 |  | 4 |  | 23 |
Unrealized (gains)/losses included in OCI, liabilities |  | 0 | [3] | 0 |  | 0 | [3] | 0 |
Purchases, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Sales, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | 38 |  | 30 |  | 38 |  | 30 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  | 1 |  | 14 |  | 4 |  | 23 |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] | 0 |  |  |  | 0 |  |  |
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value liabilities |  | 291 |  | 342 |  | 347 |  | 366 |
Total (gains)/losses included in earnings, liabilities |  | 10 |  | 3 |  | 2 |  | 53 |
Unrealized (gains)/losses included in OCI, liabilities |  | (3) | [3] | 10 |  | (41) | [3] | 3 |
Purchases, liabilities |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  | 0 |  | 3 |  | 0 |  | 7 |
Settlements, liabilities |  | (4) |  | (10) |  | (14) |  | (21) |
Sales, liabilities |  | 0 |  | 0 |  | 0 |  | (60) |
Transfers into Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of Level 3, liabilities |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | 294 |  | 348 |  | 294 |  | 348 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  | 8 |  | (11) |  | (4) |  | 32 |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] | (2) |  |  |  | (39) |  |  |
Currency Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | 14 |  | 11 |  | 8 |  | 17 |
Total gains/(losses) included in earnings, assets |  | (2) |  | (3) |  | 4 |  | (9) |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Sales, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value assets |  | 12 |  | 8 |  | 12 |  | 8 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | (2) |  | (4) |  | 4 |  | (9) |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Other Assets [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value assets |  | 14 |  | 15 |  | 18 |  | 14 |
Total gains/(losses) included in earnings, assets |  | (1) |  | 2 |  | (5) |  | 3 |
Unrealized gains/(losses) included in OCI, assets |  | 0 | [1] | 0 |  | 0 | [1] | 0 |
Purchases, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Issuances, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Settlements, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Sales, assets |  | 0 |  | 0 |  | 0 |  | 0 |
Transfers into level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Transfers out of level 3, assets |  | 0 |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value assets |  | 13 |  | 17 |  | 13 |  | 17 |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  | (1) |  | 2 |  | (5) |  | 3 |
Change in unrealized gains/(losses) for the period included in OCI for Liabilities still held | [1] | 0 |  |  |  | 0 |  |  |
Other Payable [Member] |  |  |  |  |  |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |  |  |  |  |  |
Beginning balance, fair value liabilities |  |  |  | 3 |  | 4 |  | 5 |
Total (gains)/losses included in earnings, liabilities |  |  |  | 1 |  | 0 |  | 2 |
Unrealized (gains)/losses included in OCI, liabilities |  |  |  | 0 |  | 0 | [3] | 0 |
Purchases, liabilities |  |  |  | 0 |  | 0 |  | 0 |
Issuances, liabilities |  |  |  | 0 |  | 0 |  | 0 |
Settlements, liabilities |  |  |  | 0 |  | (4) |  | (3) |
Sales, liabilities |  |  |  | 0 |  | 0 |  | 0 |
Transfers into Level 3, liabilities |  |  |  | 0 | [2] | 0 |  |  |
Transfers out of Level 3, liabilities |  |  |  | 0 | [2] | 0 |  | 0 | [2]
Ending balance, fair value liabilities |  | $ 0 |  | 4 |  | 0 |  | 4 |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  |  |  | $ 1 |  | 0 |  | $ 2 |
Change in Unrealized (Gains)/ Losses for the Period Included in OCI for Liabilities still held | [3] |  |  |  |  | $ 0 |  |  |
[1] | Reported within the “Unrealized gains (losses) on available-for-sale securities” on MBIA’s Consolidated Statement of Comprehensive Income/Loss.
[2] | Transferred in and out at the end of the period.
[3] | Reported within the “Instrument-specific credit risk of liabilities measured at fair value” on MBIA’s Consolidated Statement of Comprehensive Income/Loss.

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- Definition Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Amount of unrealized gain (loss) recognized in other comprehensive income (OCI) from liability measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Amount of increase (decrease) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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