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Investments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified |  |  | 3 Months Ended | 5 Months Ended | 4 Months Ended | 9 Months Ended |  |  |  |  | 3 Months Ended | 4 Months Ended | 5 Months Ended
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Sep. 30, 2013 | Dec. 31, 2012 | Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] | Apr. 30, 2013 Predecessor Ambac [Member] | Sep. 30, 2013 Predecessor Ambac [Member] | Sep. 30, 2012 Predecessor Ambac [Member] | Dec. 31, 2012 Predecessor Ambac [Member] | Sep. 30, 2013 Below Investment Grade Securities and Non-Rated Securities [Member] | Dec. 31, 2012 Below Investment Grade Securities and Non-Rated Securities [Member] | Sep. 30, 2013 Residential Mortgage-Backed Securities [Member] | Sep. 30, 2013 Ambac-wrapped Securities [Member] | Sep. 30, 2013 Other Asset-Backed Securities [Member] | Sep. 30, 2013 Asset Backed, Military Housing, Fixed Rate Securities [Member] Security | Sep. 30, 2013 National Century Financial Enterprises, Inc. [Member] | Mar. 31, 2013 National Century Financial Enterprises, Inc. [Member] | Apr. 30, 2013 National Century Financial Enterprises, Inc. [Member] | Sep. 30, 2013 National Century Financial Enterprises, Inc. [Member]
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of below investment grade securities and non-rated securities |  |  |  |  |  |  |  |  | $ 810,835 | $ 192,190 |  |  |  |  |  |  |  |
Gross unrealized loss |  |  |  |  |  |  |  |  | 37,209 | 43,934 |  |  |  |  |  |  |  |
Unrealized losses on securities |  |  | 101,753 | 101,753 |  |  |  | 7,551 |  |  | 21,039 | 18,369 | 23,340 | 22,876 |  |  |  |
Number of securities in unrealized loss position |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  |
Investment realized losses |  |  |  | 8,162 | 53,305 |  | 70,621 |  |  |  |  |  |  |  |  | 150,201 |  |
Investment in asset backed notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 174,500 |  |
Cash recoveries |  |  |  |  |  | 14,007 |  |  |  |  |  |  |  |  | 1,216 |  | 39,978 | 1,216
Securities fair value | 6,863 | 6,945 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of securities held by bankruptcy remote trust | 230,049 | 201,329 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments |  |  | $ 350 | $ (2,665) | $ 369 |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Amount of accumulated pre-tax unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months impacting investments. + References No definition available. + Details
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- Definition Fair Value of securities held to collateralize and fund repayment of debt issued through a resecuritization transaction. + References No definition available. + Details
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- Definition Investments in notes or bonds that are collateralized by a specific group of underlying assets owned by the issuer of the notes or bonds which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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