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Fair Value Of Financial Instruments (Narrative) (Details) (USD $) | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2012 | Mar. 31, 2011 | Dec. 31, 2011 | Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | $ 67,400,000,000 |  |  |  |  |
Financial guarantees, interest payments | 9,114,000,000 | [1] |  | 8,984,000,000 | [1] |
Percentage of sector-specific transactions valued using BET model | 8.00% |  |  |  |  |
Percentage of transactions valued using corporate spreads used | 47.00% |  |  |  |  |
Percentage of transactions valued using spread benchmarked from most relevant spread source | 45.00% |  |  |  |  |
Percentage of transactions valued using WARF-sourced and/or ratings sourced credit spreads | 78.00% |  |  |  |  |
Net insured derivative liability | 4,500,000,000 |  |  | 4,800,000,000 |  |
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability | 4,400,000,000 |  |  | 5,700,000,000 |  |
Percentage of level 3 assets at fair value in total assets measured at fair value | 29.00% |  |  | 30.00% |  |
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value | 76.00% |  |  | 77.00% |  |
Level 3 analysis, assets | 3,937,000,000 |  | 4,439,000,000 | 4,188,000,000 |  | 4,467,000,000
Level 3 analysis, liabilities | 7,622,000,000 |  | 11,821,000,000 | 7,938,000,000 |  | 9,916,000,000
Transfers into Level 3 | 28,000,000 |  | 41,000,000 |  |  |
Transfers out of Level 3 | 42,000,000 |  | 237,000,000 |  |  |
Transfers into Level 2 | 42,000,000 |  | 237,000,000 |  |  |
Transfers out of Level 2 | 28,000,000 |  | 41,000,000 |  |  |
Transfers in or out of Level 1 | 0 |  |  |  |  |
Net unrealized loss related to transfers into level 3 | 10,000,000 |  | 1,000,000 |  |  |
Net unrealized losses related to the transfers out of level 3 | 3,000,000 |  |  |  |  |
Net unrealized loss related to transfers out of level 3 |  |  | 1,000,000 |  |  |
CDS Contracts [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | 60,600,000,000 |  |  |  |  |
CMBS And Commercial Mortgage Collateral [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | 21,500,000,000 |  |  |  |  |
Required Payment Of Lost Principal [Member] | CDS Contracts [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | 45,900,000,000 |  |  |  |  |
Required Interest Payments [Member] | CDS Contracts [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | 14,700,000,000 |  |  |  |  |
BET Valuation Model [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Percentage of insured credit derivatives, fair valued using the BET model | 79.00% |  |  |  |  |
Direct Price Model [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Percentage of insured credit derivatives, value under Direct Price Model | 21.00% |  |  |  |  |
Interest Rate Contract [Member] |  |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |
Gross par outstanding on insured derivatives | $ 6,800,000,000 |  |  |  |  |
[1] | - Represents contractual principal and interest payments due by the issuer of the obligations insured by MBIA.

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