Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	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
Percentage of loss reserves and unearned premium reserve required to be invested in qualifying assets	50.00%
Statutory net loss	$ 202	141
Statutory capital	273	476
Policyholders' surplus	106
Contingency reserves	167
National [Member]
Insurance Regulations And Dividends [Line Items]
Dividends from subsidiaries	81	134
Statutory net loss	82
Statutory capital	2,000	2,400
Policyholders' surplus	1,500
Contingency reserves	$ 445
Statutory net income		$ 39