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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

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