Fair Value Of Financial Instruments (Narrative) (Details) (USD $)	12 Months Ended
	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	$ 74,200,000,000
Percentage of transactions valued using corporate spreads used	51.00%
Percentage of transactions valued using spread benchmarked from most relevant spread source	42.00%
Percentage of transactions valued using WARF sourced and/or ratings sourced credit spread	80.00%
Net insured derivative liabilities	(4,800,000,000)		(4,400,000,000)
Impact of including nonperformance risk in valuation of pre-tax net insured derivative liability	5,700,000,000		12,100,000,000
Percentage of level 3 assets of total assets measured at fair value	30.00%		24.00%
Percentage of Level 3 liabilities of total liabilities measured at fair value	77.00%		78.00%
Transfers into Level 3	9,000,000	[1]	390,000,000
Asset/Liabilities Transfers, net	456,000,000
Transfers out of Level 3	391,000,000		562,000,000
Transfers into Level 2	391,000,000		562,000,000
Transfers out of Level 2	456,000,000		390,000,000
Net unrealized losses related to the transfers into level 3	2,000,000		10,000,000
Net unrealized gains related to transfers into Level 3	32,000,000
Net unrealized gains related to the transfers out of level 3			120,000,000
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	76.00%
Percentage of sector specific transactions valued using BET model	7.00%
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	24.00%
Credit Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers into Level 3	8,000,000	[1]
Interest Rate Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers into Level 3	1,000,000	[1]
Insured Derivatives [Member] | MBIA Corp. [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross par outstanding on insured derivatives	3,800,000,000
Insured Derivatives [Member] | 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	22,900,000,000
CDS Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross par outstanding on insured derivatives	67,100,000,000
CDS Contracts [Member] | Required Payment of Lost Principal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross par outstanding on insured derivatives	50,200,000,000
CDS Contracts [Member] | Required Interest Payments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross par outstanding on insured derivatives	16,900,000,000
Interest Rate Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gross par outstanding on insured derivatives	$ 7,100,000,000

[1] Transferred in and out at the end of the period.