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Insurance Regulations And Dividends (Narrative) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
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] |  |
Statutory net loss |  | $ 323
Statutory capital | 464 | 492
Policyholders' surplus | 237 |
Contingency reserves | 227 |
Statutory net income | $ 107 |
MBIA Corp [Member] | Loss Reserves And Unearned Premium Reserves [Member] |  |
Insurance Regulations And Dividends [Line Items] |  |
Percentage of loss reserves and unearned premium reserve required to be invested in qualifying assets | 50.00% |
MBIA Corp [Member] | Policyholders Surplus [Member] |  |
Insurance Regulations And Dividends [Line Items] |  |
Unassigned surplus | $ (1,800) | (1,800)
Statutory surplus negative impact |  | $ 112
Maximum percentage of statutory net admitted assets invested in subsidiaries |  | 60.00%
National [Member] |  |
Insurance Regulations And Dividends [Line Items] |  |
Dividends from subsidiaries | 118 | $ 118
Statutory net loss | 321 |
Statutory capital | 2,800 | 3,500
Policyholders' surplus | 2,200 |
Contingency reserves | $ 594 |
Statutory net income |  | 192
MBIA UK [Member] |  |
Insurance Regulations And Dividends [Line Items] |  |
Statutory loss recorded on adjustment of carrying value to fair value less costs to sell |  | $ 114

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