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Business Developments And Risks And Uncertainties (Narrative) (Detail) shares in Millions, $ in Millions |  | 9 Months Ended |
--- | --- | --- | ---
Oct. 05, 2015 USD ($) | Sep. 30, 2015 USD ($) shares | Dec. 31, 2014 USD ($)
Business Acquisition [Line Items] |  |  |
Number of operating segments |  | 3 |
Cutwater sale effective date |  | Jan. 01, 2015 |
Mbia Inc [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Gain on sale of subsidiary |  | $ 24 |
Liquid assets |  | 306 | $ 498
Amount released from tax escrow account |  | $ 228 |
Treasury stock shares acquired | shares |  | 7.6 |
National [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Impairment of real estate |  | $ 4 |
Treasury stock shares acquired | shares |  | 8.0 |
National [Member] | SubsequentEvent [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Dividends from subsidiaries | $ 114 |  |
Warburg Pincus [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Common Stock Shares Sold | shares |  | 45.7 |

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