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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ / shares in Units, $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Jan. 19, 2016 USD ($) | Dec. 31, 2015 USD ($) $ / shares shares | Dec. 31, 2014 USD ($) $ / shares shares | Dec. 31, 2013 $ / shares | Feb. 29, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |
Number of operating segments |  | 3 |  |  |
Cutwater sale effective date |  | Jan. 01, 2015 |  |  |
Treasury stock shares acquired | shares |  | 39,900,000 | 3,300,000 |  |
Shares repurchased under share repurchase program, average price per share | $ / shares |  | $ 7.6 | $ 10.45 | $ 0 |
PREPA [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Outatanding bonds |  | $ 8,100 |  |  |
Mbia Inc [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Gain on sale of subsidiary |  | 24 |  |  |
Liquid assets |  | 416 | $ 498 |  |
Amount released from tax escrow account |  | $ 228 |  |  |
Treasury stock shares acquired | shares |  | 7,600,000 |  |  |
Mbia Inc [Member] | SubsequentEvent [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Amount released from tax escrow account | $ 105 |  |  |  |
Mbia Corp [Member] | SubsequentEvent [Member] | Zohar [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Insurance par exposure |  |  |  |  | $ 776
National [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Impairment of real estate |  | $ 4 |  |  |
Dividends from subsidiaries |  | $ 114 | $ 220 |  |
Treasury stock shares acquired | shares |  | 8,000,000 |  |  |
National [Member] | PREPA [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Insurance par exposure |  | $ 1,400 |  |  |
Warburg Pincus [Member] |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Common Stock Shares Sold | shares |  | 45,700,000 |  |  |

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