Debt (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014		Jan. 16, 2008
Debt Instrument [Line Items]
Aggregate principal amount in excess of stated amount	$ 10
Surplus notes outstanding	940	[1]	$ 940	[1]	$ 1,000
Surplus notes purchased at par value	13
Senior notes at par value owned by National	136
Surplus notes repurchased at outstanding par value	47
Long-term debt	582		592
MBIA Inc Senior Notes [Member]
Debt Instrument [Line Items]
Accrued interest	$ 8		$ 8
MBIA Corp Surplus Notes [Member]
Debt Instrument [Line Items]
Long-term debt, interest rate	14.00%
Interest rate terms	interest rate of 14% until January 15, 2013 and thereafter have an interest rate of three-month LIBOR plus 11.26%
Debt instrument, base reference rate	three-month LIBOR
Debt instrument, basis spread on variable rate	11.26%
Medium Term Notes [Member]
Debt Instrument [Line Items]
Annual interest rates, minimum	0.00%		0.00%
Annual interest rates, maximum	6.40%		9.08%
Weighted average interest rate	2.42%		2.44%
Medium Term Notes [Member] | Global Funding Limited [Member]
Debt Instrument [Line Items]
Par value redeemed	$ 40
Weighted average percent of par	100.00%
Investment Agreement Obligations [Member]
Debt Instrument [Line Items]
Annual interest rates, minimum	4.40%		3.48%
Annual interest rates, maximum	6.89%		7.38%
Weighted average interest rate	5.56%		5.50%

[1] Callable on or after January 15, 2018 and every fifth anniversary thereafter at 100.00.