Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Insurance Regulations And Dividends [Line Items]
Percentage of policyholders' surplus	10.00%
Percentage of adjusted net investment income	100.00%
Policyholders Surplus [Member] | Minimum [Member]
Insurance Regulations And Dividends [Line Items]
Policyholders' surplus	$ 65
MBIA Corp [Member]
Insurance Regulations And Dividends [Line Items]
Statutory net loss		$ 323
Statutory capital	464	492
Policyholders' surplus	237
Contingency reserves	227
Statutory net income	$ 107
MBIA Corp [Member] | Loss Reserves And Unearned Premium Reserves [Member]
Insurance Regulations And Dividends [Line Items]
Percentage of loss reserves and unearned premium reserve required to be invested in qualifying assets	50.00%
MBIA Corp [Member] | Policyholders Surplus [Member]
Insurance Regulations And Dividends [Line Items]
Unassigned surplus	$ (1,800)	(1,800)
Statutory surplus negative impact		$ 112
Maximum percentage of statutory net admitted assets invested in subsidiaries		60.00%
National [Member]
Insurance Regulations And Dividends [Line Items]
Dividends from subsidiaries	118	$ 118
Statutory net loss	321
Statutory capital	2,800	3,500
Policyholders' surplus	2,200
Contingency reserves	$ 594
Statutory net income		192
MBIA UK [Member]
Insurance Regulations And Dividends [Line Items]
Statutory loss recorded on adjustment of carrying value to fair value less costs to sell		$ 114