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Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net |  | $ 683 | $ 220 | $ 123
U S Public Finance Insurance [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net |  | 499 | 74 | 5
Second-Lien RMBS [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net |  | 62 | 121 | 63
First-Lien RMBS [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net |  | 92 | 41 | 34
CDOs [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net |  | 10 | (32) | 29
Other [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | [1] | $ 20 | $ 16 | $ (8)
[1] | Includes non-U.S. public finance and other issues.

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- Definition Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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