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Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses) (Detail) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net |  | $ 72 | $ 94
U S Public Finance Insurance [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net |  | 77 | 11
Second-Lien RMBS [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net |  | 2 | 23
First-Lien RMBS [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net |  | (12) | 57
CDOs [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net |  | 5 | 2
Other [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder Benefits And Claims Incurred Net | [1] | $ 0 | $ 1
[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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