Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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]
Unassigned surplus	$ (1,700)	$ 1,700
Percentage of loss reserves and unearned premium reserve required to be invested in qualifying assets	50.00%
Statutory net loss	$ 141
Statutory capital	476	555
Policyholders' surplus	282
Contingency reserves	194
Statutory net income		134
National [Member]
Insurance Regulations And Dividends [Line Items]
Dividends from subsidiaries	134	108
Statutory net loss		27
Statutory capital	2,400	$ 2,500
Policyholders' surplus	1,900
Contingency reserves	485
Statutory net income	$ 39