Fair Value Of Financial Instruments (Fair Value And Carrying Value Of Assets And Liabilities That Are Disclosed At Fair Value But Not Reported At Fair Value On Company's Consolidated Balance Sheets) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Variable Interest Entity [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Variable interest entity notes	$ 7,094		$ 8,697
Long-term debt			360
Value Disclosed At Fair Value Not Recorded At Fair Value [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Other investments	9		11
Accrued investment income	51	[1]	63	[1]
Receivable for investments sold	159	[1]	32	[1]
Total assets	3,086		3,595
Investment agreements	1,254		1,853
Medium-term notes	1,068		1,187
Securities sold under agreements to repurchase			286
Long-term debt	1,109		1,117
Payable for investments purchased	150	[2]	3	[2]
Total liabilities	6,747		8,111
Gross	866		1,451
Ceded	88		94
Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Variable interest entity notes	3,166		3,297
Investments held-to-maturity	2,867		3,489
Long-term debt			368
Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Quoted Prices In Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Accrued investment income	51	[1]	63	[1]
Receivable for investments sold	159	[1]	32	[1]
Total assets	210		95
Payable for investments purchased	150	[2]	3	[2]
Total liabilities	150		3
Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Securities sold under agreements to repurchase			286
Long-term debt	1,109		1,117
Total liabilities	1,109		1,403
Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Other investments	9		11
Total assets	2,876		3,500
Investment agreements	1,254		1,853
Medium-term notes	1,068		1,187
Total liabilities	5,488		6,705
Gross	866		1,451
Ceded	88		94
Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member] | Variable Interest Entity [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Variable interest entity notes	3,166		3,297
Investments held-to-maturity	2,867		3,489
Long-term debt			368
Carrying Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Other investments	9		11
Accrued investment income	51	[1]	63	[1]
Receivable for investments sold	159	[1]	32	[1]
Total assets	3,234		3,949
Investment agreements	993		1,578
Medium-term notes	1,438		1,491
Securities sold under agreements to repurchase			287
Long-term debt	1,836		1,840
Payable for investments purchased	150	[2]	3	[2]
Total liabilities	7,885		9,502
Gross	720		1,305
Ceded	94		104
Carrying Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Variable interest entity notes	3,468		3,943
Investments held-to-maturity	3,015		3,843
Long-term debt			$ 360

[1] Reported within "Other assets" on MBIA's consolidated balance sheets.

[2] Reported within "Other liabilities" on MBIA's consolidated balance sheets.