Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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)	$ (1,800)
Percentage of loss reserves and unearned premium reserve required to be invested in qualifying assets	50.00%
Statutory capital	$ 555	464
Policyholders' surplus	356
Contingency reserves	199
Statutory net income	134	107
National [Member]
Insurance Regulations And Dividends [Line Items]
Dividends from subsidiaries	108	118
Statutory net loss	27	321
Statutory capital	2,500	$ 2,800
Policyholders' surplus	2,000
Contingency reserves	$ 522