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Investments - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 9 Months Ended |  | 3 Months Ended | 5 Months Ended | 9 Months Ended |  | 4 Months Ended |  |  |  | 3 Months Ended | 4 Months Ended | 5 Months Ended | 9 Months Ended | 0 Months Ended | 4 Months Ended | 6 Months Ended | 9 Months Ended
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2014 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2013 Successor Ambac [Member] | Sep. 30, 2014 Successor Ambac [Member] | Dec. 31, 2013 Successor Ambac [Member] | Apr. 30, 2013 Predecessor Ambac [Member] | Sep. 30, 2014 Below Investment Grade Securities and Non-Rated Securities [Member] Successor Ambac [Member] | Dec. 31, 2013 Below Investment Grade Securities and Non-Rated Securities [Member] Successor Ambac [Member] | Sep. 30, 2014 Residential Mortgage-Backed Securities [Member] Successor Ambac [Member] | Sep. 30, 2014 Ambac Wrapped Securities [Member] Successor Ambac [Member] | Sep. 30, 2014 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member] | Sep. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member] | Apr. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member] | Sep. 30, 2013 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member] | Sep. 30, 2014 Gains (losses) on securities held as of reporting date [Member] Successor Ambac [Member] | Dec. 06, 2013 National Century Financial Enterprises, Inc. [Member] | Apr. 30, 2013 National Century Financial Enterprises, Inc. [Member] | Jun. 30, 2013 National Century Financial Enterprises, Inc. [Member] | Sep. 30, 2014 National Century Financial Enterprises, Inc. [Member]
Investment [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of below investment grade securities and non-rated securities |  |  |  |  |  |  |  |  | $ 240,497 | $ 826,969 |  |  |  |  |  |  |  |  |  |  |
Gross unrealized loss |  |  |  |  |  |  |  |  | 11,202 | 36,946 |  |  |  |  |  |  |  |  |  |  |
Payment of permitted policy claim (percent) | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Future payment of permitted policy claim (percent) | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized losses on securities |  |  | 15,198 |  |  | 15,198 | 116,634 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment realized losses |  |  |  |  | 8,162 | 29,401 |  | 53,305 |  |  |  |  |  |  |  |  |  |  |  |  |
Cash recoveries | 7,760 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,441 | 39,978 | 39,978 | 0
Securities fair value | 6,766 | 6,799 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fair value of securities held by bankruptcy remote trust | 0 | 240,150 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other investments |  |  | 700 | 350 | (2,665) | 5,905 |  | 369 |  |  |  |  | 329 | 1,577 | (78) | (784) | 5,965 |  |  |  |
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments |  |  | $ 31,485 |  |  | $ 31,485 | $ 116,634 |  |  |  | $ 8,608 | $ 6,805 |  |  |  |  |  |  |  |  |

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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 Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale securities impacting investments. + References No definition available. + Details
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- Definition Percentage of the Segregated Account of Ambac Assurance's permitted policy claims that are to be paid in cash from July 2014 forward as approved by the Rehabilitation Court. + References No definition available. + Details
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- Definition Percentage of the Segregated Account of Ambac Assurance's permitted policy claims that were paid in cash from September 2012 through June 2014. + 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 The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + 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 210.7-04.3(a)) -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 325 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6485959&loc=d3e22529-158502 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 3 -Subparagraph a -Article 7 + Details
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