Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	1,900	$ 1,900
Statutory net loss	129	202	$ 141
Statutory capital	134	273
Policyholders' surplus	97
Contingency reserves	37
National [Member]
Insurance Regulations And Dividends [Line Items]
Dividends from subsidiaries	60	81	134
Statutory net loss		82
Statutory capital	2,000	$ 2,000
Policyholders' surplus	1,600
Contingency reserves	402
Statutory net income	$ 55		$ 39