Insurance Premiums (Roll Forward Of Premiums Receivable) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019
Insurance Premiums [Abstract]
Premiums Receivable, Beginning balance		$ 249		$ 296
Premium Payments Received		(30)		(43)
Premiums from New Business Written		0		0
Adjustments, Changes in Expected Term of Policies		(1)		(4)
Adjustments, Accretion of Premiums Receivable Discount	[1]	6		7
Adjustments, Other		(8)	[2]	(7)	[3]
Premiums Receivable, Ending balance		$ 216		$ 249

[1] Recorded within premiums earned on MBIA’s consolidated statement of operations.

[2] The change primarily relates to a reduction in installment premiums due to refunding activity and the recording of an allowance for credit losses under Financial Instruments-Credit Losses, partially offset by changes in foreign exchange currency rates.

[3] Primarily relates to the write off of uncollectible premiums and to a lesser extent realized gains due to changes in foreign currency exchange rates.