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.