Fair Value Of Financial Instruments (Gains And Losses On Fair Value Option Included In The Company's Consolidated Statements Of Operations) (Detail) - USD ($) $ in Millions		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange		$ 22	$ (61)		$ 13	$ (44)
Other net realized gains (losses)		0	34		(1)	37
Non Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange		22	(61)		13	(44)
Other net realized gains (losses)		0	34		(1)	37
Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange		13	14		17	(19)
Other net realized gains (losses)		(93)	0		(93)	28
Variable Interest Entity [Member] | Loan Repurchase Commitments [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	9	(2)		9	4
Investments Carried At Fair Value [Member] | Non Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[2]	(3)	3		(5)	6
Fixed Maturity Securities Held At Fair Value - VIE [Member] | Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	(6)	(5)		(12)	(14)
Residential Mortgage Loans [Member] | Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	(54)	(30)		(76)	(102)
Corporate Loans [Member] | Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	0	4		11	7
Other Assets [Member] | Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	0	0		0	0
Medium Term Notes [Member] | Non Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[2]	12	(19)		6	(22)
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	[1]	56	45	[3]	83	90	[3]
Variable Interest Entity Notes [Member] | Variable Interest Entity [Member] | Previously reported [Member]
Accounts Notes And Loans Receivable [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange			(45)			(90)
Other Liabilities [Member] | Non Variable Interest Entity [Member]
Accounts Notes And Loans Receivable [Line Items]
Other net realized gains (losses)	[4]	$ 0	$ 0		$ (2)	$ 0

[1] Reported within Net gains (losses) of financial instruments at fair value and foreign exchange-VIE on MBIA's consolidated statements of operations.

[2] Reported within Net gains (losses) of financial instruments at fair value and foreign exchange on MBIA's consolidated statements of operations.

[3] The Company revised its previously reported losses for the three months and six months ended June 30, 2017 of $(45) million and $(90) million, respectively, to gains of $45 million and $90 million, respectively.

[4] Reported within "Other net realized gains (losses)" on MBIA's consolidated statements of operations.